(Be aware this game is early alpha, and thus I'm reviewing my impressions on the game as of November 8, 2014.)
Interstellar Marines is a Co-Op Tactical FPS. It pits you and 3 other teammates who are, you guess it, interstellar marines, against defective robots in a battle to survive to the end by means of elimination, escaping, or simply surviving. Or, you can also go against other players in a control point-style battle.
This game actually has a good amount of realism and atmosphere which blend very well for a game like this. The game has a real dark atmosphere to it, feels genuinely creepy and lonely with only your friends by your side. The maps, for the most part, are like an abandoned and broken facility. Very dark, and your main light source would be flickering lights or your own flashlight. It really brings immersion into the game, which is very helpful. The animations of the marines too are very well done, especially in first person. You actually see your arms move as you look around, see your own feet, and so on. Small detail, but very welcome.
Speaking of realism, that's one of the best parts of the game. Whenever you take damage, your hearing is impaired (which is important in this game), your HUD disappears until you heal, and team damage/killing is also real. The guns aren't all that powerful either, but that's a good thing. The robots you face are very powerful, and can down you in seconds. You only have two guns: One for short range, and one for long range. Neither have that much ammo in it's clip (Probably enough for 3-4 robots if you're accurate), or have amazing accuracy, so you need to be precise and careful; Spraying and gunning your way through just won't succeed.
Teamwork is heavily emphasized here. It's only four of you, and because the robots are so powerful, you're likely to go down. The only way you can survive is if your teammates revive you in your 15 seconds of last breaths. The aforementioned guns are weak with only one person, so if hoards of robots come, you all need to use your ammo and shoot tactically.
There's currently 5 game modes as of this review, and they all have nice variety to the game. Elimination is killing all the robots in the area, Escape is moving to the top of the facility while surviving from the bots, Survivial is activiating several objectives and then escaping (with robots.), and Stronghold is the PvP game mode, 2 teams trying to capture all points in the area. The other mode is a campaign mission, titled The Neurogen Incident. It appears to be story oriented, but I cannot currently play it as it crashes my game.
All in all, Marines is an incredible team based tactical shooter, especially for being early access! And I would really recommend this to anybody who wants some good times with their friends, using teamwork to achieve the goal in a dark and desolate enviornment.