After taking a look at the lore, here we are with the rules.
Whereas the recent Codex: Imperial Guard (and to me personally that army is still the Imperial Guard, no matter what Games Workshop says) featured their forces as a sledgehammer, the role of the Tempestus is more one of a scalpel. With fewer, far more elite, troops, iron discipline and rapid assault vehicles, more than a few players seem to be referring to them as a poor man's Elysian Drop Troops. While that might be true to a point, it's none the less filling a niche option within the army usually reserved for supplements with similar troops from one iconic army being taken in a new direction. In many respects, this is honestly a supplement done right without the many failings often found within those books.
The first major improvement of note is that there is are scenarios, no Planetstrike stratagems, no shoehorned efforts to force players to buy more rulebooks and a clear focus upon vanilla 40K. While there might still be padding here and there, just see the examples in the lore section, there's at least some indication that the design team aren't using this to farm even more cash from people. The only definite bonus the army keeps is one which actually saves the player some cash. The two formations they army can take are already present within the book rather than kept on Dataslate Formations. Already that's a few major gripe dealt with from similar mini-codices, and things keep looking more and more promising from there.
Unlike the supplements, there is much more of a sense of getting a more unique army. Even ignoring the Scions and army specific models, the various rules and unique squad details make this feel like a separated army. Not simply a version of the Imperial Guard with a few new toys and a fresh coat of paint. The most general of these are that all models within the army come with Deep Strike and Move Through Cover special rules, making them extremely mobile and something more akin to the Tau Empire than many other forces. Better yet atop of all this, they do not have to pay the usual tax to be scoring units. As such, in the hands of the right person they're equally effective in both slugging matches and objective based scenarios.
These basic attributes would usually be something to criticise as it might make the army too easy to use, but there are a few distinct elements which do help to balance out this force. Foremost among these is the limitations when it comes to units. Being more like special forces than a sledgehammer, the soldiers in this army are elite. They're the sorts deployed against specific targets rather than frontline troops, and you soon begin to realise that they rely heavily upon successes in initial strikes and keeping up a continual momentum. While they army is tougher than the basic Guard forces, they can't take quite so many hits and can be blunted if approaching the wrong enemy the wrong way.
Take for example the new Taurox Prime APC, which might as well be this army's strength's and weaknesses personified. The vehicle can move quicker than Chimera transports, is more accurate and is far better armed than its contemporaries. At the same time however, the vehicle has some very specific weaknesses which can easily be exploited, namely its armour. At 11/10/10 it's not exactly the most durable vehicle to ever enter the battlefield, and it's long sides mean that it easily runs the risk of being flanked and taken out. Players need to be careful about where it is placed, think about their actions and use their army's best attributes to avoid getting hit too often. Not, as can unfortunately be the case of a few certain armies, rush forwards and rely upon sheer raw power to win the day.
What also separates it from the Imperial Guard is the lack of platoons, instead having choices for footsoldiers consisting of Command Squads and Scion Squads. The former are the HQ choice along with Commissars, and serve in a largely similar role to most Imperial Guard command units. Consisting of one officer and a group of other Scions, he has the ability to give one command per turn to other Scion units within the army. As you would expect, the Command Squad can take a banner, vox-caster and medi-pack as upgrades, along with up to four special weapons from their armory.
The orders in question consist of a variety of good choices, nothing especially outlandish but something which can give the right squad the bonuses they need in a desperate situation. Three focus purely upon enhancing ranged firepower, offering Rending (against vehicles or Monstrous Creatures), Sniper or twin-linked bonuses to the squad they are given to. Two others offer Crusader to assist at close range, and Preferred Enemy which is always a nice temporary bonus. If there is a serious criticism to be made, this is a little simplistic and the only non-killing option is the ability to give a unit Fleet briefly. A few more interesting choices would definitely be of benefit, enhancing gameplay in other ways to make things interesting, or perhaps simply bolstering Leadership. Then again, that last one is what the Commissars are for.
Speaking of Commissars, the psychotic political officers are effectively what you would expect here. They remain as effective as ever at leading squads, counting as Independent Characters, having Stubborn and with a Commissar Lord being an especially tough foe with three Wounds and a WS/BS of five. However, they are a definite problem in one respect due to the Scions' lack of disposable troops. Every one they headshot in the name of the Emperor is felt much more keenly to a unit due to the much smaller squads, meaning they are effective but potentially costly to you.
Along with Taurox Primes are Valkyrie AACs, which remain unchanged from the latest Guard codex. However, they are far more important given the Scions nature and a definite necessity for any army wanting to make the best use of their strengths. After all, the Scions work perfectly well as Air Cavalry and their ability to rapidly deploy and hit hard make these things perfect for quickly reaching objectives.
For the Scion squads themselves we have what you would expect - small somewhat fragile groups of heavy hitters. Coming in basic groups of five soldiers, they come with the option to take vox-casters and up to two special weapons from their armory (flamer, grenade launcher, hot-shot volley gun, meltagun, or plasma gun) meaning players can offer a high output of firepower. Better yet, combined with their ability to Deep Strike means players can send in small squads armed with two plasma guns or two meltaguns to cause havoc with enemy transports. Even without that they're a solid option at BS 4, with carapace armour and AP 3 basic guns, so with some luck they can hit hard and take moderate amounts of fire in the right place. For what they offer, they're also fairly reasonably priced, even in comparison to the old Storm Troopers.
Now, you might be noticing something here from what we've covered thus far - There's no special characters. Much like the lore, the rules place emphasis upon the army as a whole and troops rather than a handful of named heroes, meaning that for once we actually have an army being treated like a proper army. While one or two certainly wouldn't have gone amiss, it's a sign that the creative team might be considering some of the worse effects of recent years. A big improvement given the general apathy usually seen from the company.
The lack of characters also means that Wargear is devoted to units and helping with certain strategic movements more than just a lot of stabby weapons or HQ only options. In fact, a great many are offered to vehicles. Chief among these is the Auger Array, which is a 25 point drop beacon which can be used within 6" of the vehicle it is with. Something which can make it invaluable for rebelling major offensives against your lines or when more bodies are needed to slow a foe down. Vehicles also gain a fair number of interesting bonuses with Relic Plating giving one Adamantium Will. Recovery Gear allows a vehicle to ignore Immobilised on the roll of a D6. Finally, Fire Barrels inflict D6 S4 AP5 hits on the first enemy unit to charge it.
While all of this is decent if uncomplicated, it does seem to have the obvious problem of a lack of new items for infantry or commanders. The codex also keeps a lot of what the Imperial Guard offer their units, but it is definitely a shame that the codex doesn't offer a little more original material for them.
Finally, the two formations on offer are Airborne Assault Squadron and Ground Assault Formation. These require a Commissar, a Command Squad, three squads of Scions and Valkyries/Taurox Primes respectively. Besides the usual reserve roll of counting as a single unit when they arrive, the former gains re-rolls on any kind of Grav-Chute deployment along with Split-Fire and Twin-Linked as they first come down. The latter meanwhile gains Pinning and Twin-Linked for their opening attacks when they do the same. This definitely seems like the weakest point of the book as it makes initial strikes a little too easy, and while they can offer the much needed momentum the army can use to win games, it seems like too simplistic a way to do so.
Besides the above, the biggest problem with the army is definitely a lack of variety when it comes to units. Yes, they are offered a good number of weapons and this is a mini-codex, but for what the designers were going for it seems there should have been one more squad. Something either to serve as advanced scouts or additional specialists of some kind to help offer a little more tactics and variety. This might have been intentional to try and push using the allies list more, but even then the Tempestus lack the ability to be a unique detachment, meaning that's a wasted of an allies slot.
Also the book does still carry a few current problems despite being a major step in the right direction. The structural problems when it comes to separating out model shots, lore and rules remain a big issue. What's more is that the book lacks any mention of the Force Organisation Chart, which seems like an effort to push Unbound Armies on players.
While hardly bereft of any failings, Codex: Militarum Tempestus is definitely a major improvement over some works we have seen of late. This is definitely the standard supplements and mini-armybooks should be held to, balancing solid lore with good rules. The rules themselves are hardly complex but they do encourage players to focus upon risk management and tactical advantages over unit spamming, sheer power and many negative aspects. Combined with their semi-fragility and high maneuverability, the book seems like something halfway between the Imperial Guard or the likes of the Eldar or Tau Empire. In that respect it could be seen to encourage players to pick up more xenos forces as secondary armies over Space Marines; and let's face it, after the fifth edition we seriously need more variety beyond the astartes.
The book is definitely overpriced at £30.00 for what it offers, but then what do you expect from Games Workshop, and it's far from a disappointment. If you like what you see from this book and are considering getting a new army, this one comes with a recommendation. It won't suit everyone and it won't offer the broadest of tactics, but at the same time what's there is pretty damn good. It might finally be a sign of hope that things will take a turn for the better with the game.