I may be the last to the party, but I just unlocked Forza 4's Warthog autovista and I'm loving it. Walking around the Hog in the virtual garage may be the closest I'll ever get to a real one. It feels so big and beefy and having Cortana narrate the various features was a fun touch. In some ways there's so many great details on this model that I wonder if the real ones do it justice.
Maybe the best / saddest part though was firing up the engine and hearing it rev. It had a deep rumbling sound that *some* newer Hogs lack! ;)