Well, the original Halo had a very very very annoying SP. Don't get me wrong, the story was nice but hey, whoever was the level designer for Bungie at that time deserved to be fired. Most...repetitive...SP...ever...
I've heard that the SP portion of Halo2 is also repetitive(I haven't played that yet since I do not own an xbox and need to go to a friend's house to play).
MP wise the game is ok. I still prefer to play UT2k4 but Halo 2 is pretty fun too. Well, my friend owns me 'cause he's used to playing with the controller while I'm not which is annoying but hey.
Overall, I must say I would play both. I consider Halo to be overrated, not bad. I'm mostly a PC FPS player so my experience in them is very different and some might say a lot better.
In conclusion, the Halo serie is the BEST CONSOLE FPS. Port it too PC and it doesn't last long.