i get the impression that the su does what a lot of people on this thread seem to be looking for...as far as working with loops goes, it's MUCH better than the sp-303, 505, 808, asrx, or the mpcs, for that matter.
...it's much like a hardware version of acid, with assignable knobs, ribbon controller, really good filters and fx...the midi implementation sucks, but i have mine running as master clock to the mpc1k...su runs loops, and the mpc does everything else...tight.