Usually, it's something so sick and depraved that you can't get it out of your head for weeks. Other times it can be poetic in a haunting way. And sometimes it's just downright clever.
It's art. It's poetry. And in a weird way, more honest and sincere than something you'd find in an art gallery. It's anonymous, so people can really let their ego come out. Bathroom art is the human psyche at its most raw.
I'm going to start collecting photos of bathroom graffiti. I think it would make an intriguing coffee-table book. We need some sort of open-source network, like a wiki, so anyone and everyone can post their own pictures.