The adventure does what it says on the tin: the Gem Prison is a place to escape once you're in it, providing a break from more traditional dungeons. If that's what you're after, this is worth your time.
On first reading I quite liked it; having now run it, and while I still recommend it, I see a weakness I didn't at first.
The thing is quite clearly a puzzle dungeon, and just about any group will start writing down the room glyphs and trying to decode them. So far, so good, though it won't actually get them out of the dungeon.
But the fact that it's obviously a puzzle dungeon brings me to my biggest gripe. There's a puzzle in room 10 that, as written, isn't actually solvable. It was once, but was broken by previous adventurers. Which at the table is a giant waste. It's a timewaster and red herring in an adventure that's already encouraging the players to slow down, write down glyphs and solve puzzles. It adds nothing to the game except penalizing the players for playing along and trying to grapple with the dungeon. Nor is it even obvious how the puzzle was supposed to be solved before it was broken, which would have made the whole thing immediately salvageable at the table.
So that's one strike against it. I don't hold the single exit against it, given it's explicitly a prison, but consider warning your players in character that it is a prison before they go in rather than springing it on them. There are a couple of ways past that single exit and its guards, which I appreciate. There's also a notable treasure in there, that my players missed entirely in choosing not to fight a powerful foe, another grace note I appreciate.