David Eppstein has an analysis of the SoCG author feedback with respect to his papers. Worth perusing: his overall conclusion is that having the rebuttal was a good idea, but he'd like to hear from the committee (perhaps at the business meeting?).
I had two papers rejected. For one there was no feedback requested, and the paper was rejected. The final reviews made it clear that the reviewers understood the main contributions of the paper - what was under contention (among other things) was how the material was presented, and that's obviously not something author feedback can help with.
The other paper had one request for feedback which was basically a long negative review, again focusing on the presentation. We tried to respond as best we could, but it didn't make too much of a difference.
He did concur that the quality of reviewing was very high.