I bought this game over 3 years ago on a promise and a lot of hyperbole about what the developers wanted to achieve. The product is not yet finished and it will be a little while longer before we have a full single player campaign. However, what is there is very good, by no means perfect, but very good none the less, and I have no hesitation in recommending it in its current state.
* Lots of different game modes, with the main game being team based PvP, but also a 4 player Coop story mode vs bots, and various other co-op player vs bots modes.
* The AI on the bots is fun to play against and they offer a genuine challenge, and have on more than one occasion made me jump.
* Very focussed gameplay, there are only 2 weapons and limited ammo in each clip, so every shot counts
* Great community of people playing, there is quite a lot of chatter on the servers with players, and people are generally quite nice, funny, helpful.
* Quick loading times to get into games
* Great respawn idea, basically when you die you have to wait 2 mins before respawn, but if your team captures zones you get to respawn earlier. This makes the game quite tactical and differentiates it over your usual COD, counterstrike type gameplay.
* The game can be tough for beginners, but experienced players can be quite helpful, so stick at it and it will come to you
* The developers are occasionally found playing with/against you, this gives the feeling that they also enjoy playing the game and adds an extra bit of fun when they do.
* Day/night mode on maps; the night mode makes things especially interesting, how often do you use the flashlight to see where you are going vs giving your position away. The positional sound of footsteps also can give you away / help you find people.
* A bunch of other things I can't be arsed to type.
* Despite selling a lot of copies of the game, and there being quite a lot of servers, only 1 or 2 of the main ones tend to fill up. However, once you are in at the right time of day the games are fun, in fact I have had quite a few 2v2 or 4v4 games that were amazing.
* Lag. This to me is the games biggest issue, and can be quite frustrating at times, however, again with experience you can compensate for it by firing ahead of target. If there is one thing that needs some more TLC its the lag compensation and reducing overall lag when connecting to servers.
* I play a lot of counterstrike and miss not being able to melee people, i would take anything over nothing, even if you had to charge melee up before striking. The prestige of being able to sneak up on someone and stab them in the back and embarass them would make this game twice as good.
* Not much else other than its not finished, but like I said this is still a very good and polished game
Overall if want something a bit different then this game might be for you, also you can test some of the early stuff out on their website before committing: http://www.interstellarmarines.com/
or even watch a few videos of it on youtube: https://www.youtube.com/results?filters=hd&search_query=interstellar+marines&lclk=hd&spfreload=1
edit: added link