I've once considered it, but the public bounties that I could find were so laughably low that I just saw no way to make it work. <$500 for a bug that would take at least a week to fix? I guess it's a nice tip if you were going to work on it anyway, but it makes no sense for me to schedule time to work on that bounty when pretty much any other job would pay 5 - 10 times as much.
I guess it could work if there were a lot of bounties for a project that you are familiar with, so that you can fix them quickly, but there are so few bounties available that you probably have nothing to do after a week or so.