I'm using a Pintech ConcertCast set I got off ebay for $500 in excellent shape... (sells for $800-900 new)...
I just got it recently and last time I looked there were other sets with just the triggers for pretty good prices... got it off ebay...
works much better and much faster than trying to program the MIDI track by draging the notes with the mouse... gives the drums a much more realistic feel and sound too...
the MIDI out of the drumsets on the D4 are GM compatible, so pretty much you just plug the right triggers into the right input (how the trigger guide tells you to) and go...
the 12 inputs are nice on the D4... I'm only using 8 right now but plan on adding 2 more cymbals and at least another tom...
I just wish the D4 could alter the MIDI notes... since the drum sets I have for battery have the bass drum on 2 notes, snare on 2 notes, and the 2 main toms on 2 notes... I didn't see anywhere in the manual where it could alter the MIDI data randomly between 2 notes... anyways it still works great...
P.S... I was gonna build my own triggers but went with the Pintechs instead... they give a great response...especially the snare trigger... snaps like an authentic snare.. the toms and snare are about 2 - 3 inch deep wood shells w/ silentech heads on them which keeps them silent when hitting them, but gives a more natural (or realistis) response from the drums since they're not just pads but heads on a wood shell...