I just got one of these (Gecarcinus quadratus) they seem to be quite easy to keep and quite interesting. I do not believe that they eat just shrimp....from what i understand they are scavengers and eat just about anything .....and this does seem to be true. You do have to supply some calcium for them however.
Here is a link to where i got my info from. It even shows how to set them up in a tank.