Pony Island is pretty interesting. I don't really know what else to say about it.

The gameplay of the actual "Pony Island" part of the experience is not really all that fun. It's tedious, repetitive, and a bit boring at times, but it's necessary for the game to progress.
The puzzle solving aspect is a lot better, and the twists the game introduces on top of that system are really good.
Finally, everything else the game throws at you is phenomenal. There's not a lot I can say about this part of the game without spoiling it, but suffice it to say that this is the part of the game I stayed for. I especially one part of the game at the very end, where it uses clever tricks to confuse you.

All in all, Pony Island is a pretty good game, but is dragged down by its insistence on making you play the actual game.