I did this ride a few weeks ago, but just got around to the ride report today. First off: the whole report with routemaps and pictures.
Old Waterman Canyon is a great climb with almost no traffic and a stout grade. If you do the 18 climb, I would highly recommend replacing the first part of 18 with Waterman.
18 itself is quite scenic and you get some great views:
18, however, carries a lot of traffic at 55mph. Even early on a Sunday morning there was a fair amount of traffic. Lighter and slower traffic and this would be up there with GMR.
138 is an insane piece of road from 18 to Crestline:
Old Waterman and 138 I would definitely recommend. 18 I would avoid at most times - just too much fast traffic and a huge speed differential when ascending. Still glad to have done it though.