Rayman Legends did it utterly brilliantly with the Murphy levels. Particularly the endless challenge missions with two people are fantastic. It's completely different from anything else I've experienced, too.
In a similar way, I also loved how it was used in New Super Mario Bros U and Super Luigi U. I have a friend who I play 2-D platformer games with, but after a while it becomes too difficult for her, and it removes the fun. With these games, she played as far as she could manage, and then swapped to the gamepad.
I also played it with 4 people who aren't very good at platformer games, while I aided them with the gamepad, that was a blast too.
In general, I'm very happy with it when it's just used as a secondary screen, though. The information you get in Hyrule Warriors, for instance, is very pleasant, and avoids cluttering the main screen more than necessary.
