This was my first time doing Mt. Diablo. I went with a friend who's done Mt. Diablo a few times before. I did this route, which takes suggestions from previous posters on this forum: Mt diablo Walnut creek to Pleasanton BART at Bikely.com
I arrived at Walnut Creek BART ~ 9:30 am. It was pretty easy to find California Blvd. The only tricky part was the right turn from Newell onto the Ironhorse trail. It's a very sharp turn, so I had to get off my bike to walk a few steps up the sidewalk. Ironhorse at this time was not too crowded but we went onto Danville Blvd. Danville is a great road since the bike lane is quite wide and the traffic is not too bad. We were passed by a few cyclists who actually didn't seem to be heading to Mt. Diablo. The left turn onto El Cerro Blvd was not as difficult as I expected, but you'll have to squint to barely make out the street sign and then head for the left turn lane. It was probably just after 10 am at this time, and traffic was still not too bad. El Cerro to Diablo Rd wasn't great since the bike lane goes away, but we didn't have any issues.
The left turn onto Mt. Diablo Scenic comes a bit unexpectedly since there's no sign indicating this is the way up Mt. Diablo until after you've turned left. There is no bike lane but the traffic is much lighter on this road. I estimate that 90% of the people on this road and all the way up to Mt. Diablo summit are cycling. Another pleasant surprise is that I didn't encounter any motorcyclists. The best surprise is that the water was running! I expected that the water fountains were off because of the drought. We refilled our water bottles at the S. Gate ranger station and again at the Junction.
The last couple miles to the Summit were the hardest because it was really starting to warm up. When on the right side of the mountain, there was no breeze but it was a relief every time we rode on the left side and could feel a decent wind. I had to walk up the steepest 200 yards to the summit. I had a good laugh when someone passing me said that he'll probably be joining me in walking soon, but he actually made it all the way up by doing some S-turns.
The water at the summit was again available and so were the bathrooms. The way down was fun although there are a few very sharp turns at the beginning of the descent to N. Gate after the junction. I took every turn expecting to see a car barreling up, so I slowed to < 10 mph on most of the turns so I could hug the right side of the road, but the traffic was surprisingly light. The path from N. Gate back to BART was great. N Gate Rd, Walnut and Bancroft have very wide bike lanes, and it's extremely obvious when to turn on the route I linked above (thanks to the posters here who suggested this).
If I were to do this route again, I would try to leave even earlier, but I already got lucky with the weather conditions for late June. S Gate is definitely easier to climb because there are more flat areas and way more shade.