You can drive on close to 95% of the original in CA.
Although there are some questions on which streets it really followed in the immediate Los Angeles area, west of Monrovia/Duarte, Huntington Dr to Foothill to Colorado and then the route from Pasadena into Los Angeles. Different sources show different route, but all the streets and the Pasadena Fwy are there. It's all available from Duarte to San Bernardino It's almost all available from Berdoo to Needles. A small gap in Devore where the I-15 crosses and then allthe way to CA-138 and then covered by the current I-15 to the top of Cajon Pass, and then it's almost all there until you get to the border. IIRC, it's almost all available in AZ, although there may be a few small interuptions by I-40
Some of the landmarks in CA urban areas are still there, A Tepee Motel in San Bernardino, urban sprawl has caught up west of there. The problem with east of Victorville is all of the small gas stops are gone until you get. Barstow retains a very significant 66 character as it is about the only outpost of "civilization" before you get to Vegas on 15 or Needles on 40. East of Barstow, you need to make sure you keep gassed up at the I-40 stations where 40 and 66 come together because you could have a stretch of about 120 miles with no gas stations sticking to the old road.
I hope you like desert because from Victorville to Amarillo that's about all you will see. I-40 and the dieselization and improvements of the Santa Fe RR killed off a lot of the smaller towns/wide spots in the road. A lot of those "towns" served both the RR and 66 as water and fuel stops, and once they weren't needed they dryed up.
It's a neat drive, with a lot of other stuff to see, Grand Canyon, Painted Desert, Las Vegas and Hoover Dam aren't all that far off the 66 route.