Chris has posted a 10 minute video strutting some of the nice features of developing with Trails (built on Ant, Tapestry, Spring, XDoclet, and Hibernate). This was inspired by the now famous Ruby on Rails video.
In theory I like the concept that you can prototype a web application and then fill in the details such as unit tests, dao layer, business logic, business facades, and web layer as your needs for complexity grow. Other solutions that I've seen in the past that attempt to provide this type of RAD environment ultimately seemed to hinder flexibility later in the project. With Trails that doesn't appear to be the case though.