I myself haven't tried JBoss Errai at all, but I am thinking of migrating the server-side services of my current GWT project into a new project that will be now treated as a platform.
The services in the currect project that are accessed via RPC can be commonly used with other projects as "base platform."
So, I am thinking of using using JBoss Errai for this scenario. And so, the new "base" platform project will be non-GWT and that Ajax apps built with GWT will just call into this platform for authentication, storage, etc.
Is this a good approach anyway?