I love the warthog in Halo. It's just fun to mess around with when you're doing some split screen multiplayer.
HOWEVER
In general, I could do without vehicles in shooting games. I hate vehicle based levels, I hate vehicle whores when playing online. At most, vehicles should be used as methods to get from point A to point B (which is how it was mostly handled in Halo 1 with the exception of the tank sequence in AOTCR, not so much how it was handled in 2 and 3). Combat should be strictly an on-foot activity.