Is it wrong of me to say a little of both? I think in some ways CM Punk is both underrated and overrated at the same time.
When he first came into the WWE, everyone, who was a fan of his ROH stuff, talked like he was the second coming or something like that, and then when he debuted, it was very lackluster. Sure, it was ok, but nothing huge like some people were expecting. I personally blame the WWE approach to things, and it probably killed a lot of his offensive moves that he did in the indies, but still, I think he came in very overrated.
Now, however, I think, in the eyes of the WWE, he is underrated. He puts on good matches, he's decent on the mic, and the fans are behind him. However, with WWE, he's almost like a throw away guy that they can do stuff when they want to, but don't have to keep doing stuff with him. They seem to do that with a lot of the younger guys these days. Kofi Kingston is another example of this. Now with the Money in the Bank, I really hope they continue to build him, and make him into the Main Eventer I think he can become.