Timeshift is the prime example of a good concept (time manipulation) badly rendered. The story in itself is utterly slim and unattractive (and with quite the erratic narrative) : a scientist, Krone, decied to found his own empire in an alternate timeline and you go after him. The end. No surprises, no twist, no incentive to continue.
The hero is a soulless mute, joining the Resistance just because and being ordered around, while nobody bats and eyelid at a suit guy appearing from nowhere and able to manipulate time around him.
There's a grand total of two timejumps !
But well, story is not meant to be the strongest point of a FPS. Problem is, the gameplay is also lacking. Not that it plays badly- on the opposite, it plays quite smoothly. But the game is far too easy. Your ability to slow time, combined with the quick regeneration of your health and a genous cooldown for time-controlling abilities, makes you near-invulnerable if you stay cautious. The game compensates by unleashing hordes of brainless soldiers, but it just makes the fights longer, not more interesting.
The puzzles are short of anything creative. A puddle of electrified water ? Stop the time, the water is solidified, go unscathed. The bridge is collapsing ? Activate return, walk on it as it goes back in time for your needs. It won't challenge your intellect, rest assured.
The weapons feel good, especially the explosive crossbow, but in the end you feel like you spend most of your times pushing buttons and going back. Cooke, the leader of the Resistance, yelling at you every twenty seconds in some sections till you accomplish the objective is quite unnerving.
The game is mercifully rather short- about 8 hours, without replay value. In a way, it's quite soothing because you stomp the ennemies with your arsenal and slow time capacity, but it was really clunky and uninspired at the time, it's not better now.
If you want a FPS with the Time them and a good dose of humor, try Time Splitters : Future Perfect. Else, even Singularity is more convincing than Timeshift : at least you interact with the world, you're not a mute puppet in a suit.
Surely Saber Interactive is doing better now, but then they have also produced Battle Los Angeles... Not the brightest past.