We've just finished Route 66 from California to OKC (have to head for a wedding in East Tennessee now...). As stickdog mentioned, most of it is under I-40 or other highways now. Those parts that still exist are fine for the RV (except for an extremely old part through Oatman, AZ, over the mountains). Unfortunately, there's not much left except for a lot of kitsch and curio shops. We ate at several neat diners and got a lot of photos of old motel signs, etc. It's nothing like I remember from the late '50s and early '60s with my grandparents, though. As stickdog also mentioned, LOTS of motorcycles doing the Route. We encountered several large groups of Europeans and Scandinavians who had rented "the whole experience" complete with Harleys, support vehicles, mechanics and guides.
Rob