Tumbling is plagued by a horrible tragedy of the commons problem: the more tainted your coins are, the more you want to tumble them. People who legitimately acquired their bitcoins don't need to tumble them, so chances are you are just getting other tainted coins in return.
People seem to think that coins are 'clean' after tumbling, but I think its much more likely that you'll get back more suspicious coins: superlatively, "Here's my personal-use drug bitcoin, give me coins that were earned by assassinating someone"
