The first Halo game is one of the best games ever made and has gone on to something approaching iconic status. Its greatness is established in my mind by the fact that, as a FPS, it just plays better than anything else out there. In terms of balance (two weapon system and regenerating health), still brilliant enemy a.i, and the sheer variety of the action (on foot and vehicle 3 way battles), it really was a special game.
Halo 2's single player felt a little lost by comparison. Whilst the core gameplay was still there, the levels didn't have the same epic feel and the story tried to tell too much at once. The MP on the other hand was as good as anything that had ever been played online.
Halo 3 found itself in a rather invidious position therefore. Sandwiched between those people who were just going to hate the game no matter what (I've yet to see a game series generate so much hate from those who profess to have little interest in it) and those who wanted Halo:CE mark II. In the end I think they delivered the game of the generation thus far with perhaps only Galaxy having reached the same level of greatness for me. Barring the Cortana level it was an epic single player that, for me, really shows up the likes of Resistance et al as also-rans.
So yeah, I'd say I'm a rather big fan of the series.