We just recently completed our first Personas project for a customer and are having the handover today. It was quite an interesting project and we learned a great deal. Working with our customer we were able to significantly simplify and consolidate some screens, improving productivity and reducing the amount of user training required. It's an excellent fit for modifying and consolidating SAPGUI screens to enable better workflows.
Like any product, there are some limitations. We found a few frustrations with tab ordering and exposing F4 help simply in a custom text box (it can be done, but it is quite an effort) but it was overall solid. Most of all, we found that it pays to have a good design and understanding of what you want to do in the transaction and access to the people who will be using it so you can do quick iterations on the screens.