I dunno. Halo 1-3 felt more like Halo music to me. ODST was a bit Halo, but a bit more jazz fusion I guess. Was a neat mixture. Reach's felt... overdone, for lack of a better word.
Hmm, I think Reach's soundtrack wasn't that great to listen to outside of the game, but as far as atmosphere is concerned, it does a really good job at making you feel like you're on another planet. The colours and sounds are all just a little alien and far-off sounding, which contributed to the mood they were trying to go for. "From the beginning you know the end" type deal. The main theme percussion is kick-ass, though.
I loved the music played in New Alexandria (especially the night sections), Long Night of Solace, and Pillar of Autumn. The skyboxes and set-pieces in the game are just beautiful, and the "almost" Halo sound really added to that. Made the whole game feel somewhat sad and mysterious, despite what it was.