I have a year experience in smartGWT, when we started our first gwt lessons, they said use the basic gwt features and build own framework otherwise we can encounter errors which may lead to dead-ends. We considered the idea, but for the project (since we already had deadline) decided to look after some extension of gwt. We found smartGWT. The showcase was really convincing. Remember these are widgets, the basic logic of your system is still up to you.
In case if you start using smartGWT: If you don't use JSON but some normal list of java structures prepare to get hard hours/days to get use to datasources, grids, and few other components. Seriously, it had ridiculous problems (for example switching two configuration code line without reason makes work the listgrid), BUT before you drop the entire idea to use it, after a while it starts to work, and it becomes really impressive and comfortable. This is a kind of work "make it once, will work hundreds of times".
In my opinion:
don't start use smartGWT if you want to make a system which is not too complicated.
start use it if you need complicated system with lot of different features, if you don't have the time to develop everything for yourself: many components are ready in smartGWT.