I just did this route a couple of weeks ago. Going to Kanab (heading south for me) I drove from I-15 to Kanab using 20 to get to 89. It is a summit you go over but the road was good with passing lanes in parts. I followed a semi over it -- just take your time and don't go faster than the speed limit.
On the way back home going North, we decided to not take 20 and to stay on 89 all the way to Salina and then take 50 over to I-15. There is less elevation up/down this route and less traffic. Also, unless you drive faster than 65, the time is almost the same and it is a more scenic route. If I were driving this again, I would do go this route.
