The old adage about chess probably applies here. Mastering Rails will always be challenging as there is so much innovation in the community. Just as you have "mastered" something, someone invents something much better You can view this as a bad thing or a good thing, depending on your attitude.
With that said however, I found Rails very easy to understand and learn. I quickly built a "standard" web app in a few hours. In the Java world, you'd have to use AppFuse or some other "jumpstart" mechanism. Rails' concepts are nothing new, though many techniques are simply not possible in Java.
I feel the real learning curve will be Ruby. It's an easy language to learn, but a tough one to "master". Coming from the Java world, there are many many idioms & techniques that simply don't exist in every day Java coding. Which is why a book like Rails for Java Developers is even more important for Java developers.
Perhaps as an encore, the authors can work on "Ruby for Java Developers"