I owned 3 of the 4 expansion boards but sold them on to other owners, after 'playing' with them for a few months. While some of the sounds were quite good, some were - in my opinion - awful. Many of them seemed to be modifications of the sounds already in the KN7000 and these 'New Sounds' could often be achieved by using the Sound Editor, built in to the KN7000. My basic opinion of these expansion boards, is similar to Mark's.
Since I like to share my efforts on the KN7000 with other owners, if I used any of the expansion boards in a KN7000 recording, unless the other owners had the same expansion boards fitted, they would not be able to take advantage of the 'New' sounds and their KN7000 would just substitute one of the built-in sounds, which kind of defeats the object!
I agree with Roger's remarks about the styles mainly being responsible for achieving a good sounding arrangement and is the main reason why I usually create my own styles - particularly for the more modern songs/tunes. However, I feel that the Sounds are also very important if you are trying to achieve a reasonably authentic result - for example - a 'Hank Marvin' guitar