I have to say that this is the best Halo game that I've played.  Of course I'm a bit biased as I never really liked the prior 3 proper Halo games.  The graphics are much, much better than Halo 3 (in fact, I'd say that the graphics for Reach is what Bungie promised for 3 in the announcement trailer).  The characters are more engaging than any of the characters in the prior 3 games (except for Cortana).  And most importantly, the level design makes sense.  In all prior Halo games, several of the more open levels kind of looked the same and thus I'd get lost easily.  In Reach, the levels are more unique and easier to tell where the hell you are.  Of course, no more flood which I hated in all prior games.