One of the worst ports in gaming history.
First of all there are a few good things with the game. The first being the that the hole campaign was transfered over to the PC. Although no knew missions were added there did seem to be a lot more allies in the single player. The game also does look a bit better at the higher resolutions. They've also added online multiplayer and added several new maps.
Well that was the good things. First of all co-op play, one of the best things in the game, has been removed. The graphics do look better but they are two years old and can't compare to anything else that is out. Also the multiplayer is laggy and has become unbalanced. The first halo had great balance, but now with the inclusions of banshees and the fuel rod canon, that balance is gone. The banshee can just tap you and you are instantly killed, and the fuel rod gun is like a semi-auto rocket launcher with 4 rounds.
If you really want to see what Halo is like buy the game and have some friends over because this is not the Halo I know and love.