It seems to me that the best low cost way to learn Spring Web Flow 2.x is the book ""The Definitive Guide to Spring Web Flow"" (primarily about version 1), the reference documentation, API and sample applications.

Would you have any better suggestions?

For me, it was complicating things I was learning Spring 3 in Spring Roo style as my first version of the Spring Framework and a majority of online examples were focused on either Web Flow 1, Spring 2.5 or both. One small but nice exception is spring by example. Another was Developing Spring Web Flow 2 Application. But still, for the fresh Roo user wanting to do Web Flow 2 a lot of example translations had to be done in order to understand what's going on. This is where I found "The Definitive Guide to Spring Web Flow" invaluable as it explains very well in great detail more or less the complete framework giving the context I missed.

A mistake I did in the last half year was trying to learn web flow 2 from Spring Web Flow 2 Web Development, which initially seemed like a good idea as it's about version 2. However, I found it seriously lacking except for the testing chapter (May all java books from now on be written with tests at the centre).