We've used the underground almost exclusively to get around Frankfurt. The guy at the hotel who helped us with directions on the first day was really hinting that we shouldn't bother buying a ticket. "Go to the station, buy a ticket...or don't. You can get on without one. There's a €60 fine if you get caught, but..." *shrug*
We always bought tickets, but he was right. We never once got checked and never saw security checking for tickets.