I have given this game a fair chance (BTW, it isn't like Halo) and I think it's a bit rubbish. Certainly nowhere near the same class as the first. The framerate is very choppy meaning that FPS portions of the game play very poorly. The "stealth weapons" are nearly useless; the stunprod taking 5 or 6 goes to take down an enemy. Therefore, the game mechanics make stealth gameplay unnecessarily hard and the framerate makes the FPS gameplay poor. My advice is to either wait until a decent PC patch is released and play it on PC (if possible) or forget it altogether.
Certainly if you are looking, in terms of quality, for a Halo replacement then this isn't in the same league (and I wan't that taken with Halo to start with).
I definately think it's worth playing if you like games in this genre, but, agree with russraff that it was rather underwhelming and didn't live up to my expectations. If you've not played the first one I'd recomend you play that (£4.99 at play) or if your looking for a more modern game to tide you over I can recomend the following fps- No One Lives forever 2, Call Of Duty, Hidden & Dangerous 2 and I'm just about to receive my copy of far cry which looks very promising (93% on pczone and 91% on pcgamer)