Halo was actually the first FPS I ever played, and I loved it.
Halo 2 was good fun, but I thought that the third was somewhat lacking in the 'fun' department.
Halo Wars was a pile of tripe, with shoddy controls and some cheap winning tactics (play as Prophet of Regret, send him in to enemy base, space laser, YOU ARE WINNAR).
I still haven't plated ODST, I've heard it's good fun, a bit strange, but hey, the Halo series has always been a bit strange.
Reach was a brilliant swan-song from Bungie's part of the Halo series, and I just hope that 343 Industries carries that prowess on.
Now, there are bad bits to the series:
The online was never al that good (mainly the community, but you can't hold that against the developers), the AI could be either completely derptastic or insanely smart, but never somewhere in-between.
Some of the guns were overpowered (Halo 1 pistol, I'm staring intently at YOU) or incredibly weak (Halo 3 assault rifle, MAN UP).
Now, I shall leave and wait for Vrex to turn up and completely shatter the thread with his Halo-fanboyism.
PS: Halo Legends was pretty good.