Nice game with average graphics and cinematic. Space Siege means several hours of granted fun, but no more than that.
But there are good things in Space Siege that makes it a nice game. First, the playability is a precious mechanism of fun. Just point and shoot the bastard aliens. You can't customize your character at first, but, like the RPGs, there are several skill-trees that you can evolve the way you like. You can turn your self in a robot-being that has new skill-trees along the game, you have special power like bio-mechanicals and electric arms, a sword made by laser, but everything is from other games: the telekinetic abilities are from Mass Effect, the laser-sword is equal of the Halo 3 weapon, and the skill-trees are almost like the Titan Quest's trees.
Overall, Space Siege doesn't bring anything new. It mixes some ideas from really nice games and makes a fun game that will entertain you for several hours. But don't expect nothing more than that.