Story and Characters: Witty, lovable, and at times genuinely funny. Totally unmatched in recent memory.
Level Design: All based on psychological concepts and cliches. A few can be underwhelming but that's because the majority are truly innovative and clever!
Gameplay: Like any platformer, it can be a bit frustrating if you're knocked off a ledge and die a few times. Some enemies are overpowered and others underpowered. Item collection can slow down the pace a bit. But the psychic powers ("magic") you eventually gain mostly make up for it. I played with no problem using a mouse and keyboard.
Replay Value: Mostly if you want to re-experience the story, but it might also warrant a second playthrough if you want to be a perfectionist and collect every last hidden item.
Overall: Totally worthwhile. The story, characters, and level design make it worth all the praise. I'm frothing for a sequel!