The northern section of 66 (twistiest portion) was recently repaved in 2014. They did a pretty cheap job so there is a little bumpiness, but compared to what it was 2 years ago, it´s practically perfect. Once you hit the newly repaved section you won´t find a straight section of road for more than a few hundred feet at a time.
Corners are NOT all marked, and almost none of them have a ´suggested´ speed limit posted. Just because there is no speed marked on the corners absolutely does NOT mean you can take them at the road´s over-all posted speed limit of 55 MPH. (at least not and survive)
Lots of blind or at least reduced visibility corners, especially when foliage is on the trees.