I have been through Frankfurt several times consisting of short layovers with just enough time to pop into the toilet, grab a soda and then rush to the next gate for take off. Recently I had an 8 hour layover and decided to get out of the airport and check out the city. At first, I was thinking that Frankfurt should be renamed Frankfurtistan, but once I got out of the airport I changed my mind.
If you want to get a bite to eat or have a walk about you can pop down to the railway station in the basement