I like Ambrose, and I think he'll be a top guy one day, but he gets hyped up an awful lot. I think if he /doesn't/ make it to main eventer, people will blame WWE instead of any fault of Ambrose's.
As far as his selling... well.. Ziggler looks like a rag doll getting thrown around by a tempermental 5 year old on a sugar high, and folks want him pushed. So, I don't see Ambrose's selling turning people off from him.
I do agree with DRG, he plays his character very well. Like Cody, for instance, both are pretty masterful over any character given to them.