No problems, whatsoever. Everyone is very well behaved. If not they get a bad reference on their page and then people might not host them in future. The better your references are the more people will host you, or will want to stay with you. I haven't heard a bad story yet!

I know one guy who rents an extra flat with two double beds, just for hosting couchsurfers. He is nearly 50 and seperated from his wife, so I think he just does it to meet new people.

The Russian girl I was chatting up last night was staying with him. I told her that I'll host her the next time she comes ;-)