I was honestly getting tired of Halo around Halo 3, in fact, I was ready to say I HATE Halo. But ODST changed my mind - it showed me Bungie still had ideas, ambitions, DREAMS... And then Microsoft poured some gasoline on them and lit it up with a match.
From Halo to Halo 2, we got better AI, more vehicles, more enemies, more storyline, a new playable character, vehicle hijacking, DUAL WIELDING in an FPS for the first time (I think), and some badass new armor and gameplay mechanics and levels.
From Halo 2 to Halo 3, we got some shinier graphics (doesn't wow me very much), some cleaner, more "finale" oriented music, some epic scenes, a few new vehicles and weapons, an arguable even more disappointing ending, and one of the most agitating, publicly-rated online experiences I've ever had the chance to witness.
So when ODST came around with a new character that completely flipped the difficulty on its heel for me, I was flabbergasted to see Reach. I was skeptical at first, but it looks like they're going out with a bang. If this really is Bungie's last game, then once I've pilfered a new 360 I'll be running to go and get it.
TL;Dr: Looks awesome, once I get a 360, I'll be playing non-stop.