I have seen it go both ways.
On one hand, I have seen former Starbucks employees hungry to delve more into the world of coffee and really strive to prove themselves.
On the other hand I've also seen what others have mentioned where a person is resistant to learning a new way. The biggest problem I've seen from hiring previous Starbucks employees is a lack of quality control. They are used to things being automated to the point that they are lazy behind equipment where you have to be in charge.
Again though, each person takes their experience with the Green Mermaid differently. There are some who go on to do great things, many of whom you would never know worked there. (TX-Coffee comes to mind...)