Timeshift has a fantastic gimmick - the ability to manipulate time - but unfortunately utilizes it in a game that's mediocre in every other way. Of course, I'm not saying it's bad, I'm simply saying that the ability to control time, as fun as it may be, doesn't make this a great game.
You can slow, pause, and even rewind time to help you in your quest to take down the bad guys. These abilities are all a treat to use. However, the actual meat of the game is nothing more than a run-of-the-mill FPS. The game takes place in a Dystopian future that's very reminiscent - thought not as original and enjoyable - of Half Life 2.
There's so many good first-person shooters on the market right now, such as Bioshock and Call of Duty 4, that it's hard to find time for something less than stellar such as Timeshift.