When to use GWT and when to not

I am wondering if there are specific cases where GWT is not suitable? For example would it be approriate to re-develop Stack Overflow using GWT?

I am developing an app which has a Java Restlet API and I was planning on using GWT (previously I would just have used bog-standard HTML/CSS with back-end PHP code calling an API). I am wondering if there are reasons why I shouldnt choose to do this?