Conversation so far:
What is JEM?
Vai designed the Ibanez JEM guitar, a unique instrument that incorporated a series of groundbreaking designs.
What was unique about it?
The guitar was designed with a sharper body shape, a large cutaway around the neck, twenty-four frets, a fully floating tremolo system.
What was different about its sound?
new low-range sonic possibilities.
What was his connection with Universe Guitars?
Another design by Vai was the Ibanez Universe, the first production seven-string guitar, which was released in 1989.
Did he design any more guitars?
CANNOTANSWER.
Are there any other interesting aspects about this article?
Since its original 1987 release, the JEM has become one of the longest-running and most successful signature series guitars in history.

Question: How popular was it?
Self-contained rewrite: How popular was the Ibanez Universe guitar?