I finished Halo yesterday (in order to be up-to-date when the 2nd part comes out). I had given it a go when it came out on the PC, but that was just running painfully slowly and ended being not much fun. On the Xbox however (strangely) that is not the case (if you ignore the last bit of the last level which is chugging quite heavily).
It has great music and great art-direction (with some cleverly used cube / environment maps), a good AI, nice physics & vehicles and a decent story. It also has some repetitive levels in the middle section and some vehicles (Scorpion) are underused. The gameplay decision to only be able to carry two weapons at a time makes for some interesting choices and definitely adds to the game.
Overall, I’d have to agree with one of the video interviews of the Bungie-people stating that “Halo” is basically 2 minutes of fun over and over again. Getting into close-quarter fights with the Covenant or the Flood, throwing some grenades and seeing the bodies fly is simply good fun. 😀