I will answer again, hopefully I will not be moderated out again.
Most of 66 is quick photo opportunities. You need to be on your toes,and often parking is non existent. Some towns have really done a great job of capitalizing on the Mother Road mystique while others apparently do not see the value.
Much of 66 is now the service road for I-40.It is a pain to drive but do not succumb to the interstate, there is much to be seen.
Most of the attractions have the same old 50's and 60's kitsh so don't expect anything new and wonderful.
The story of 66 is the story of the nation, modernization, urban renewal. It is replete with the successes and the (many) failures and the results on unintended consequences. A sobering ride and a chance for reflection.