I'm an excited and rather proud owner of a new MD-UW, so I haven't had a chance to sequence it externally yet. However, I've been dreaming of buying something fancy like a P3 sequencer, as I think it would do a few rather nice things that the internal sequencer couldn't. The one I really want is the Signal Arts MAPS sequencer. Check it out:
http://www.signalarts.ca/Both the P3 and the MAPS seem more like instruments in themselves, and go someway towards filling the gap left (prematurely?) by the Notron.
dg