Halo was a good game. Too bad the two sequels weren't that great. I think the only reason I picked up 2 and 3 was, well for 2, the improved gameplay, and 3, for the online junk (I didn't have xbl on my regular xbox and well--okay I picked up H3/XBL due to a boy)
I'm hoping ODST is as good as everyone was saying its gonna be, cause I was really looking forward to it.
For the series as a whole, the story is decent, gameplay is okay, but the music is divine. Great OST. But yeah, when it comes to online FPS, TF2 > Halo.