Having played Halo 1-3, I have to say that they are all crap, in my opinion. They add nothing to the genre (other than that they DID make an FPS playable on a console, which is something, though I still prefer PCs).
My problem with Halo is my problem with Half-Life. They are old, bland, and generic FPS games.
Half-Life was a 1999 game that has remained a 1999 game in style through episode 2.
Halo is a 1995, Doom-style, FPS with very little else to commend it. Oh, and it has horrific vehicle sections with goddawful controls.
No, overall FPS's haven't evolved much, but some have tried more than others, which is why I dislike the genre, but do enjoy some games that stand out for me: COD4 (not MW2); FEAR 1; Portal and TF2 and L4D1/2