I've got one of the jam man ones, the basic old version, you can play a sequence, phrase, etc doing a bit of rather awkward foot tapping on the pedals to start and stop "recording" your riff or whatever.. You can then play over it, and record more layers if desired.
I've just used it very occasionally, more just to try ideas out over a basic sequence/pattern type of thing, tho folk do some very impressive stuff with them.. There's lots of YouTube vids going into these in depth.