I've been on 11:00 Saturday tour recently and really enjoyed myself! It was a good time as it wasn't too busy yet in the area. The guide, Ben Slow, was great, passionate about what he does. I even enjoyed the 2 minutes of "educational moaning" about gentrification, especially when some guy in his pajamas looked out from the balcony of one of those "generic monsters" as the guide was pointing at it and shouted "I completely agree, but..."
I thought the tour was informative enough to awake one's curiosity and make one find out more about the street artists we've seen. I had another walk round after the tour and kind of got stuck in the area for the most of the day.
In a nutshell - highly recommended!