I've just started to play around with Eclipse GMF.
Has anyone used the framework? Yes, I am using it right now. It works, but it is typically quite a bit of coding for the graphical figures. I currently am struggling to leverage the IBM RSA/RSM UML editparts/figures/nodes etc built on top of GMF.
Any good or bad experiences you made using it? Looking back on my initial dives into GMF/EMF/GEF etc I can say for certain, study the examples. There are important patterns that you have to pick-up on from the examples and not the documentation. I would also suggest a new book (Eclipse Modeling Project: A Domain-Specific Language (DSL)) specific for GMF Modeling in Eclipse. I paged through it and it seemed to be the missing manual to some of the more basic concepts. Why the book is good is that is focuses on the key to making UML/Models useful through constraining it to a specific domain and providing a tool that only allows for valid models to be created. There is not a lot of documentation online and the API only tells you so much. WATCH OUT for repaint/paint loops caused by calling setBounds() or other set methods on children, it crashes the eclipse instance, not fun. Oh yes and the APIs are split between eclipse help documentation versions or not included at all.
Any alternatives for graphical modeling you could suggest? Consider UML profiles with custom images and icons rather than full shape generation. It takes about 2 hours to put together a pretty good Image/Icon editor building on top of UML graphical objects and UML profiles. The IBM RSM tool UML Profile tooling project does this quickly. There is a lot you can do with constrained UML profiles (via Eclipse plug-ins or OCL). Entering GMF land is more than a order of magnitude effort increase, from 1 to 10 hours no problem.
Consider pure DSL (Domain specific language) tools out there. Google will provide a good list. From what I have seen the main reason to use GMF is eclipse integration and leveraging existing ecore/UML models, this is why I use GMF.
Ask yourself do I need model which is easy, or do I need a tool for creating instances of this model. If there are only 1-5 expert users there may not be a need for a sleek tool.