Timeshift takes a little getting used to, mastering the few timeshift facilities (slow, stop and reverse) but once you've got used to them it's good.
Quite a few interesting little puzzles where you have to use those facilities.
Looking for another fps now so FEAR 2 may well fit the bill.
Although as mentioned earlier I still have Perseus Mandate to complete and also about half of Crysis Warhead.
I'm quite sure I've played all the other fps's I have.
Also just realised I haven't tried Steam's 'Portal' yet, which came with the Orange Box set, I don't even know what it's all about, will have to try that thing