I literally just finished it last night. So here are my thoughts:
1) I love a good FPS game. XIII is good, but not excellent, but does have flashes of brilliance. I just recently played the old PS1 Medal of Honor, and while it looks like ass, MOH is a better game. I do enjoy both single and multiplayer, with no pref for either -- I always play thru the single player portions of any FPS game.
2) If u are a heavy PC FPS player, it is worth $9.99 on the PS2, but not for the multiplayer. It is built using one of the excellent Unreal engines, but the multiplayer needs mouse capability like nobody's business. It is fun, without a doubt, and I have been playing it against the bots, who are quite good, but the aiming is just not as good as TimeSplitters 2. If I had a PC, I would go with the PC version because it will support 3rd party maps.
3) I really enjoy how the game looks, and what it is trying to do. Adam West's voice overs were great, and Duchovney was not as bad as most reviewers claim. The story is well supported by finding certain documents, so take the time to go thru and read everything you find between game sessions to immerse youself deeper into the game.
I rate it a C+ to a B-. I really like what the single player portion is trying to do, but I find the number of available multi-player maps to be very average, and the multi-player is hamstrung by its lack of mouse support, especially if u are used to PC FPS gaming. And then sometimes the single player is just more FPS gaming, like everything else. Early on, I was disappointed, but I kept at it and really enjoyed myself, esp after I got half way thru.
It's worth $10 (CC), but TimeSplitters 2 is a much better console FPS game (than just about any other one).