I've just got back from a fantastic presentation on Spring Roo from Jan Machacek of Cake Solutions. It was hosted at Skills Matter by Barry Cranford of the London Java Community. I came away totally understanding what Spring Roo is for.
In a few words, it's code a generator that gives you a boost when creating Java and Spring MVC based web applications. It uses a lot of Aspect Orientated Programming (AOP) to take away a lot of the boiler plate code, such getters and setter on beans and persistence, that are required for CRUD web applications. If, after the application is completed, you want to stop using Sprig Roo, you can. If you want to keep using it to generate more of the application, you can and it examines the code every time you build to see if it can generate more helpers from your changes. It's not a silver bullet or a golden hammer thought, it's another tool in a Java web application developers arsenal.
One draw back for me is that Spring Roo requires Maven (quite possibly one of the roots of all evil) and I don't yet know know enough Grails to know if Spring Roo provides greater advantages for web development.
Jan Machacek is a excellent speaker, very clear and enthusiastic as well as knowledgeable without being arrogant. He clearly understands Spring Roo inside out and was able to answer all of the questions asked. The only frustrating thing was that every time I thought of a question, it was answered a few slides later.
You can see the video here.