Halo has always been a mainstream title, which is to say, none of it was all the intriguing. The story line was a more or less standard affair, however, you did notice how much work Bungie puts into the game, with good looking environments that call for exploration, i.e. the Skulls.
I've found the multiplayer fun, but usually I've had the same problem as it, as I do with Counter Strike, being that when people play it for too long, they become too good, and being beaten into the dirt every time I re spawn gets very boring and usually results in a rage quit.
However, that's not to say I didn't enjoy my time with the game. I did immensely, and I still rank it as one of my all time favorite LAN Party games. But, single player was always lacking, and it was arguable as full of testosterone as the multiplayer.
With ODST, they gave us a fresh look at the Halo world from Random ODST X's perspective, as well as R-ODST-Y, Z, and !, which was great. It was fun, but still accessible to Halo-newbies.
Reach, so far, seems like an almost entirely new spin on the series. Classes with actual useful differentiations between them was something I was not expecting. Another thing was the Airborne combat, which adds a new sense of exploration, and is just cool. I don't know how I feel about Reach though, there are some grenade balance issues right now, and Bungie seems to have thrown at least some of the Cannon out the window in favor of making a better game (Shielded Armor instead of SPI for the Spartan IIIs).
TL;DR - I'm ranting so I'll wrap it up. Halo has been fun, but not groundbreaking. Reach looks good aside from a few balance issues with Grenades and it's defiantly on my "games-to-play" list. Bungie has been trying to escape Halo, but Microsoft knows a Cash-Cow when they see one, and like to milk them for everything it's worth... I don't want to see that happen to Halo.