I recently sent my second-hand 50-500mm EX back to Sigma as it wasn't focusing quite right. I called them a few days later to see what they were going to charge me for calibration or whatever. The rep said that it was being repaired under warranty. That was good. 18 days after Sigma received the lens I had it back and found it to be much better at getting good sharp focus. It'll never be as good as my Canon 300 f/4 IS L but then I didn't expect that.
Needless to say, I'm quite happy with Sigma service.
