Solid at least. Really good programming and puzzles. One VERY frustrating part in Atlantis, but aside from that, amazing and more or less 100% fair.
Also, very beautiful game without sacrificing old-school ZZT visual or play aesthetics. The best of many possible .ZZT worlds.
It's good enough I'm considering trying again for a second ending or more secrets, because I think I missed at least one ending component and I KNOW I missed at least one secret.