I think the expansion boards (at least the ones I own) sound very good, much better than the factory-installed patches on my XP-80. I own the Vintage Synth, Session, Vocal, and Orchestral boards, and have no particular favorite board because they each fill a different musical need (style, genre, whatever...). I don't know how well they compare with the sounds of the proud, expensive Kurzweil synths or other such synths. If you own an XP or a JV synth, they are a convenient and fairly inexpensive way to pick up some good sounds.