I have a Boss RC-50.  My mate has one too.  He's done a lot more with his
than I have with mine to be fair.  Playing in a rock band as I was when I
bought it, we needed an easy way of interjecting sounds like police sirens,
thunder etc... into songs we were covering, and by pre-recording these into
the RC-50 it was simple to just step on a pedal at the right time, so my
unit has really done more of that type of thing, which is not necessarily
what it was designed to do.   However, the RC-50 is very good for solo
performers to simply lay down layers of backing parts in real time and then
to play/sing over the top.  The results can be very effective and
impressive.  There is of course a bit of a learning curve in getting to
grips with the RC-50.  On the down-side, synchronising with external MIDI
units like drum machines can be problematic with the RC-50, and as I
understand it Boss don't really appear to be very interested in resolving
those issues.  Pity.

I got mine on-line from Brixton Music Exchange, which at the time was
selling the RC-50 new for a lot less than other on-line stores.