In the winter, the experience is much more enjoyable. Temperatures hover in the 50's, the fields are green, and the waterfalls are in full effect.
The route up Mt. Diablo takes the better part of a day - if rest stops are a priority. The surrounding hills were shrouded in mist - it felt like we were in the Andes looking over the jungle. There was even a rainbow sighting!
The great part about reaching the Mt. Diablo summit is less adventurous folks can drive [or bike] to the top for the same views!