I have the Super Sound Set, which I got mainly for the harpischord and organ pipe sounds. To me, no synth is complete without a harpsichord--those were always the best on the old analog synths. The Super Sound Set also rounds out the drums, and has some nice strings, and more guitars. The quality of the sounds is comparable to what's already built in--that is to say, high-quality Roland sounds. Roland included a CD with recordings of all the expansion cards with my XP-80. I bet they would send you that CD just for the asking.

I got my card in Japan on a business trip for about half of what you could get it for in the US. Cheap airfare from the US is $450 right now, so if you bought three cards, you would come out about even, and get a trip to Japan out of the deal!