Not such a big deal of a climb, I did it to the summit on a street bike after eight hours of shoveling horseshit as a job at one of the horse stables on the mountainside when I was a teen just for something else to do at the end of the boring work day. Three or four times over a three month period.

The road is paved and you can do the circle through Concord and Danville or vice versa. It's a great three or four or five hour ride, but not hell unless it's a hot day and you don't bring drinking water.

The view at the top is a nice reward, it's the highest point for hundreds of miles in every direction and on a clear day you can see the ocean, the Sierra Nevada, and north to the Yolla Bolla Mountains, and the fast gentle switchback ride down the hill into Diablo Valley is an equal or even better reward.

As far as cycling in California goes, though, Mt. Diablo is an intermediate ride, nothing like the radical backroad or single track mountain bike cycling available in the Sierras or Death Valley or some other Californian zones.