My 311 experience in Houston.
1) I noticed a traffic light that was working erratically at an intersection that is know for having many accidents. I called 311 and reported it and got a call back very quickly asking for confirmation of the location and problem. I drove back through there a few hours later and the problem had been fixed. I don't know if it was fixed as a result of reporting it, but was happy it was fixed.
2) Driving back streets in a low income neighborhood to avoid traffic, I noticed water bubbling out of the street at an intersection. There was enough water on the street that I figured it had been that way for a while. I reported it via 311. A few weeks later I came back through there and noticed the water was bubbling out of the street at a higher rate than before and there was nothing to indicate anyone had visited the problem. I reported it again and about a week later someone fixed the leak.
In both cases I wasn't provided with any kind of reference number to follow up on it. It was also difficult to report intersection where the water leak happened because one of the street signs was missing.
I don't know if this information helps anyone but I thought I would share it in case it does. 311 works here in Houston although it could use some improvement.