The story doesn’t make a ton of sense--there are big gaps in the plot that are largely ignored. The wordplay was more cute than clever, mostly puns. Again, kids would like this, but it falls pretty flat to an adult. There are a lot of characters who run quickly in and out of the story without having much effect. The characters generally only have one personality trait or aspect each, and even those are not consistent. The moral(s) are generally positive, although there are also a ton of them, and they are not regularly applied by the characters. It’s not like Milo began to appreciate learning and used that to save the princesses, for example.
Overall, The Phantom Tollbooth is a collection of cute ideas that would likely appeal to children, especially if it’s being read to them in small segments. But without the nostalgia kick, it doesn’t hold appeal for adults.