I'll admit that I am not a rabid FPS player. The only reason I even looked at, and enjoyed, Halo was because I've had a raging stiffy for Bungie's games since they first started making them for the PC and Macintosh back in the 90s.
What drew me to Halo initially was the fact that it is set in the same universe as Bungie's Marathon series. My theory that it would prove just as enjoyable in the single and multi-player areas was proved correct (although I consider the story VASTLY inferior to Marathon's).
And while it is true that Halo did not introduce us to the "lone-marine-saving-the-world" gaming cliche (gotta give props to Doom and Quake for that), it certainly brought some new elements in. Mainly the regenerating shield/health thing that needs to be traded for good ol health meters.
Even though Halo did not give us an interesting main character (c'mon, he's an emotionally and personality devoid faceless suit of armor whose name is actually a military rank), it did give those who actually liked it a nice online experience and pretty solid gameplay for single or co-op play.
My eternal beef with this series, and the reason I am no longer on XBL, is the fanbase itself. As several others have noted, the fans are damn annoying. I'm not going to list why since I'd just be re-stating what has already been said.
My main point: give props to the series for what it brought to the FPS genre. To me, it's a decent FPS, not the greatest, but certainly not hate-worthy.