I've always been a big fan of Angry Birds. I like how that keep adding levels without charging.
That being said, I really get annoyed with those who charge more for the ipad version. When the ipad first came out, a lot of developers took advantage and charged $5 for an iphone game that is only $1. I'm not one to tell anyone how to run their business, but does it really cost them that much to make the game with higher graphics? I could maybe see charging another $1 to upgrade to the ipad version, but come on.
If I were a game developer I would make my apps all universal, people catch on and become more loyal when they don't feel like you are taking advantage.