Originally great game, ruined by microsoft's monopoly approach for PC.
After long waiting period it finally came to PC users, but Microsoft had to ruin it all with its monopoly approach, due to the lack of public interest in Vista, as people refer it Windows ME2 (Windows ME was total failure).
The most expensive, hardware demanding and useless operating system ever created.
Originally Halo 2 was a DirectX9 game, which means that this "Vista Only" label is nothing more than just a cheap advertising trick from MS.
I hope they get a grip on themselves and release a patch allowing WinXP users to play it too, this could help them save their reputation, and increase their sales.
Pros:
Good storyline
Pretty good multiplayer
Cons:
Vista only game = poor performance, lack of public interest
Outdated graphics
Discrimination regarding multiplayer players (Gold user > Silver user)