TL;DR: good environment and good visuals but gameplay is terrible.
The best part of this game is the space hulk atmosphere: cramped, dark, broken corridors strewn with Dark Angel and Mechanicum iconography, and so many places for tryanids to pop out. Unfortunately, it's less a mish-mash of a variety of xenos vessels, and really just a bunch of Dark Angel ships that were smooshed together, but oh well, can't have everything. The visuals are pretty good too: good textures in environment and xenos models, weapon effects, etc. Unfortunately, these two things do not a game make.
List of my biggest gripes (for single player campaign). (From what I understand, MP is basically co-op, and your AI buddies aren't the worst I've seen. Then again, objectives are not so complicated that you actually need smart AI.)
1. Extremely linear gameplay. Go here. Press a button. Now go here. Fight some xenos. Now go here. Defend. Now go here. Press another button.
2. Maps are large (well not really that big, but the fact that you have to WALK everywhere makes them seem big. no your stamina as a space marine is apparently unenhanced so you can sprint like 5 seconds. I thought getting to wear Terminator armor in the Deathwing meant you were best of the best, but whatever. maybe gravity is especially strong.), and objectives are ALWAYS at opposite ends of the map, making you waste so much time going back and forth. You would think they could cluster objectives and move you thru the map in one direction, but nooo... Also, if you don't explore the map, you'll get trapped in the maze of corridors with deadends, doors that are permanently blocked, or stairs that only have one entrance point, all the way on the other side of the objective. Of course.
3. The lack of SAVE function. For a game built for PC, you would think this would be a no-brainer. There are so many technical glitches that being able to SAVE and LOAD is a near requirement. (From buttons deciding not to work, or guns not shooting, or relics not spawning where they're supposed to, or your idiot AI buddies getting stuck somewhere, etc). The Autosave doesn't occur nearly enough, and if you go on a treasure hunt (collecting the collectibles) and happen to accidentally die, well that's it for you. Last Autosave could be the last objective you accomplished, hours ago. I mean, it IS hard to die, what with the gateway and the heals, but enemy AI definitely cheap shots you: Oh look, enemy missle launcher that can apparently shoot thru walls to hit you, but your shots can't. Or stupid gun toting xenos that apparently can make headshots while moving. Same with the xenos that spit gobs of green goo.
4. Skill tree and weapon loadouts don't make a difference to gameplay. At it's base, the game is shoot unending waves of tyranids. Which is mostly fun, but after a time you wish for some variety in how you kill them. Thinking of changing your skills or your weapon loadouts? Not going to matter. Especially at higher difficulty level. Despite the "too many to count" weapon jams, the heavy plasma cannon is the way to go, both for yourself and your "heavy weapons support AI." The other ranged weapons don't do enough damage, or force you to reload too often, or have so much recoil that you can't actually hit anything not right in front of you. The melee weapons don't attack fast enough when there's hordes of the upper level xenos and you'll inevitably get mauled to death. Or a psyker tyranid will lightning you and that's that. I tried a psyker build, but the skill CD is so long for the powerful ones that they're basically useless. 120 sec CD? seriously? and you only get 3 skill slots? There's no wave that lasts a full 2 min...
In summary, the graphics are great, the gameplay sucks. Good try, but we need better from this franchise.