I enjoyed it - Raven is pretty good at making competent, if not revolutionary games. It draws a lot from bioshock (including its use of UT), and it takess a little effort to wrap your brain around what exactly is going on (not always in a good way), but the guns felt great, the time-powers were excellent. It's nice to have another game take a stab at time travel (TimeShift being an unmitigated disaster, and Darkest of Days feeling like a poor hl2 mod). Singularity isn't an industry-changing premiere title, but its definitely a solid title worth playing... if you like time travel as a plot point, and you can tolerate some Unreal Tournament quirks. Too bad it got buried in the shuffle. I heard about it, then forgot about it, when it came out, thanks to a few higher profile games.
It's funny you just posted this now, as I beat the game all of two days ago!