Let me guess, there was no music or sounds?
No, I bought it to complete my "Games that use enhanced music when played on a Super Game Boy" collection. Also, every internal SGB sound effect is used in the game, as well as the *enhanced* music. I'm assuming it's rare for this factor, just like Gimmick! for the Famciom is extremely rare since it uses a sound chip for it's music.
Also, I made a music modifiers for tons of Game Boy games. This one is for Super Mario Land 2: 01??68A4. Have a request?