What I've read here/been told/do: "Go DOWN the mountain no faster than you went UP the mountain!" Your JakeBrake/2nd gear are your friends. When you hit the crest, find a pullout and make sure your BrakeBuddy/toad braking system is working properly.
Two things about Glenwood Canyon (I-70 west of Gypsum): The rock formations and the Colorado River flowing through them are phenomenal. Couple that with the engineering marvel that is I-70 through that area (I'm a sucker for architecture & engineering feats) and you have some incredible beauty!
While there is a lot to be said for leaving the Interstates behind, I'm more of a "Get There!" type... I reserve most of my sightseeing for the destination and decide later if I'd like to come back and explore an area on the way later. Maybe on the way home.