One of the things that strikes me about the JM4 is that it doesn't do anything that I can do with, say, Acid. I can cut sections out of songs (or just specify loop points) and loop them and, hence, jam with the Allman Brothers, Tony Levin, Kenny Burrell, whoever - without too much prep work.
Assumedly, this device is targeted at non-PC users. I considered acquiring one for about 5 minutes - until I realized I can do essentially the same thing with my PC. The M13 does look interesting though.