Brian Szabelski at Blogcritics.org gives a thumbs up to Halo 3:

Halo 3 is just about everything we've been led to think it would be. It falls short in some areas of campaign, where things seem like the generic FPS cliché of "here's a room full of bad guys, kill 'em!", but it absolutely shines in multiplayer modes. And guess what? The game's difficulties aren't just cosmetic changes like in some other titles - it's insane on Legendary, but you're rewarded more for it. No 360 owner's collection is complete without this title.

Check out the complete review for his list of pros and cons.