I think the ideal looper (in a live setting) should enable us to record about four loops such as intro,chorus,verse bridge etc.We should be able to do this without a lot of tap dancing with the foot pedal or take the hands off the guitar to fiddle with knobs etc.
Once you have loop 1 (say,intro) recorded,you hit button 2 and recording stops on the first loop and starts on the second.Same with three and four.Four loops should cover a basic progression.On most of the current loopers,you have to stop recording the first loop and then hit the second button for recording the next loop.
Once you have your progression you should be able to access any loop you want (Not necessarily in the order in which you taped it).Most of the live looping you see on the tube is stacking or layering and the backing is kind of 'monotone' after a few measures.
Now,if you can 'stack' onto each loop independently (eg-a walking base line) you have made yourself the perfect backing track to improvise over,live)
Not sure if I made any sense but there you have it

!
p.s: Still trying to figure out the 'Rang III'. May have to wait for their software upgrade in Feb. to see if does what it promises.