It won't win any awards, but it is fun while it lasts... mostly.
Timeshift has a derivative, dull, and fairly vacant storyline, and the characters are ripped straight from several games, "Half-Life" being the most stolen from. Yet where the story fails, the gameplay... meets the mark. It's lacking in variation, and the puzzles are dull, but shooting someones head off with a shotgun, then rewinding time just so you can see it again, is always fun. The combat gets extremely repetive really quick, but it's fun enough to last the duration of the game. The multiplayer is just there. I wouldn't play it, but it works.
Timeshift isn't anything special. It's an unispired game, but it's also fun when it sticks to what it's best at, mindless time alteration, and bloody combat.