I think for a lot of players it was their first real fps, what with it being a major xbox release (and up until that point, the console fps genre was pretty niche). Talking to my friends, their opinion of it ranges from adoration to indifference and it seems directly tied to the fps-es they played before and after Halo.
Personally I don't like Halo. I've never been able to get into it because it's on a console and playing shooters with a gamepad makes me physically uncomfortable. When I have played it's been to humour my friends and Its never really impressed me, just run of the mill.