i said this recently in another thread but i've done most of the really hard colorado area routes, and certainly the really tough ones like loveland, evans, cottonwood, trail ridge, lefthand, and i definitely think peak to peak is mile for mile the hardest stretch of road ive done.
its not that any one stretch is steep or long but its so variable and so up and down that when you get from one end to the other youre so exhausted... really draining.
when they talk about doing a pro Tour of Colorado again, i think having a stage that starts in blackhawk, follows P2P to estes, then climbs trail ridge for a summit finish just before the rock cut would be as spectacular as any stage in europe.