I don't do it all the time, but I have made comments to parents before on really well-behaved children.
Once we were at a pizza buffet and the family behind us had a couple kids who sat when told to sit and eat, only asked one adult for quarters to play the video games (including please) and generally seemed like good kids. Especially compared to two other tables nearby whose kids where ...less than stellar and the parents didn't even try to reign it in. Hard to explain it, but if you remember the kids' magazine Highlights that had the Goofus and Gallant examples, it was like that.
As we left, I stopped by the mom of the group behind us (there were a mom, dad, grandma and a couple uncles along with the kids - we heard some conversations to lead me to this conclusion). I told her that her kids were really well-behaved and she should be so proud of them.