I drove a good part of it, but not the whole thing. Started at St.louis – that arch is cool. Went down to TX, AZ, NM area but then got distracted by all the national parks and left the route.
Much more relaxing that the interstates – they are very busy with trucks and it is stressful to stay there for a long time – but it gets you there quick.
Often the route 66 is beside interstates, perhaps with a 65mph speed limit, a shade slower than the interstate, but then the construction and traffic slowdowns seem much less. Also easy to stop and take a pic, have a snack or whatever you want!