Sadernes is pretty much a summer venue for the Barcelona crowd. I've climbed there in July, August and September. The conditions are not ideal of course, but the river keeps temps humane most day, and swimming in the cold water mid-day helps to keep the body cool.
The hardest sends happens other times of the year of course, but it is one of the few places close to Barcelona that is decent in Summer. Sant Llorenç del Munt is another (maybe not for August though…).
Sadernes is really popular with hikers and families on vacation during the summer months.
I tried some of the steep sectors that overhangs the river, but they where really too greasy in high summer.