Many people will instantly judge this game due to it being a 'hackneyed IOS port that doesn't belong on the glorious PC engine' or complain about it's more lackluster porting issues.
As a person that's been wanting a decent 40k tactical game, however, this hits more buttons than I ever could have hoped for, considering the starvation of somewhat-cheap turn-based tactical games as of late.
STORY - 4/5
Relatively basic as far as 40K is concerned, and requires a small amount of knowledge of things in the universe to get all the offhand remarks and comments caught. To make a long story short, you're the field command of a tactical squad of the Deathwatch, a coallition of other Space Marine chapters opted in to work with the Ordo Xenos of the Inquisition. In this situation, a sector known as Astolat is under the threat of the Tyranids, essentially space locusts that eat planets bare of any bio-organic life that fuels the creation of even more Tyranids, that are slowly chugging towards the center of the galaxy. Your job: Ensure the Hive Fleet is mired here, and if possible, keep it from passing through the sector. Again, simplistic but servicable for 40k and doesn't pull a 'LOL CHAOS WAS BEHIND IT ALL ALONG!' stick.
GAMEPLAY - 3.5/5
The gameplay is still heavily based off it's IOS roots, with controls almost all capable with just Left Mouse, though some things have been tagged with hotkeys, making a chain of Overwatch setups that much more easier. Each soldier has access to 4 AP (by default) per turn, allowing them to move, shoot, or do other actions with those AP over that turn. As far as my experience with the game goes, Your marines are split into four combat classes (Assault, Devastator, Tactical, and Apothacary) who each have preferred weapons and get special skills depending on the Chapter the marine belonged to, with each portrait type denoting what capabilities and abilities that specific marine may have. Everything in the game has a 1-4 Tier rank as well, so the higher the tier, the more special abilities or stronger the thing is defaultly. The tyranid units, while more numerous in the map, tend to act like cannon fodder to a squad of well-trained marines, though in later levels you will meet tougher enemies, up to and including massive 3x3 tiled monstrosities that require heavy weapons and well-trained marines to take down, and managing where the marines are and where they're going.
PROGRESSION - 2/5
Easily the most annoying part of the game, and sadly where the port nature comes to bear the most. In order to get more wargear (and marines) you must complete missions, which will grant a random wargear item, and 20-40 Requisition Points (varies by difficulty options, but you can't do difficulty options until you finish the game's 40 or so levels once). These requisition points are used to purchase booster packs, an obvious replacement for a previous F2P cash shop mechanic from the IOS version. This also means that by the time you run into your first major hurdle (like, say, the first Carnifex fight), you may not have the right equipment to face off against it, and thus be forced to grind until the RNG gods bless you with a godly marine/weapon combo for taking down large beasts. The marine's progressions is a bit more tolerable- each marine can gain exp during missions, with most exp to an individual coming from landing killing blows (and each kill giving a small sum to every survivor as well). This banked EXP can then be used to unlock abilities, traits, and level the marine's health/accuracy/crit rate, as far as I can tell the stat buffs are unlimited, with a constantly-increasing buffer of EXP to reach per level. If a marine dies in action, you'll lose that banked EXP however, which makes really grinding a high-leveled marine's exp a bit hard if you're not just tricking a endless-spawn mission for EXP grind (Burning Bridges comes to mind...)
IMMERSION - 9/5
Why yes, I broke my scoring system for this gag, what of it. The setting and presentation of the game is easily the BEST of any 40k game released in the last 5 years, Dawn of Wars included. Viewing the world of the 41st Millenium in a tactical level is why I pine for a Dark Heresy game built like Shadowrun, and while our Deathwatch marines don't actively meet any friendlies in the adventure (as far as I'm aware), the world looks absolutely gorgeous, thanks to the use of the Unreal engine. Even the lowliest of Ultramarine Tacticals have unique visual differences over other chapters that help to show off the units, and the splendor only grows as you get higher-tier units adn the eye candy shows more. The unique chapters for the Enhanced Edition in particular are especially visually delightful, of particular note the Dark Angel Veteran, and the other unique units of the chapters ain't too far off. Another thing, something that not much of the promotional work seems to touch on, is the bantar that goes between your squadmates. Blood Angel, Space Wolf, and Ultramarine squad members will talk between each other, from jabs about chapter stereotypes to musings on if the Emperor would be unhappy about the Ecclisiarchy, to questioning if they are indeed fighting a losing battle for the planets of the sector. The only reason this wasn't marked a 10 was because the Enhanced Edition chapters weren't included in the banter.
TL;DR
It's rough around the edges, and if you are allergic to grinding you'll drive yourself nuts here, but otherwise the averageness of the gameplay tends to get overshadowed by the close-up look of Tyranid Hiveships, torn-apart cities, and corroding Imperial bunkers, and your squadmates talking casually as they mow through hordes upon hordes of the Tyranid menace. If you're looking for a turn-based tactics game where your soldiers aren't made of tissue paper, you can't really go wrong with Deathwatch.