GameSpy gives Halo 3 5/5 and calls it "the reason the Xbox 360 exists". They also say:

What was funny to us was that the staggering quality of Halo 3 is apparent in nearly every single aspect of the game, regardless how trivial. The magnificent voiceovers featured in the cut-scenes (Ron Perlman does a great job) spill over into the gameplay in the form of an all-pervasive combat chatter that will surprise you with how entertaining it is. By the time you're through with Halo 3, you'll have witnessed insane zealotry at its finest as well as the best Warthog driving level ever.

Not surprising there, as with the possible exception of Halo 2's rough edges, attention to detail is what distinguishes Bungie, and Halo, from the crowd.