Why should/would developer that already know Java, JSP/Servlets, Struts learn Rails/Ruby if there is another Java like(curly braces) dynamic language that is compiled to Java bytecode, works with other Java code and libraries...?
To me, Ruby is a very simple language in which it removes many of the headaches and annoyances from programming that other languages such as Java bring to the table. Such as declaring datatypes is made much simpler in Ruby. Ruby removes many of these slight annoyances that make it much more pleasant to write code in. Java has many many rules that can affect the outcome of code to a beginner or even some advanced programmers that are not aware of these rules or idiosyncrasies of the language but in Ruby mostly everything works the way it is supposed to even for the beginner.
I guess what I'm saying is that Ruby is so natural that you spend your time not thinking of how to code, implement, or get something working with the language that you are using but spending your time actually getting ideas from your head to the computer in a fast and efficient way.
I've just started with Ruby and Rails but I'd say a likely candidate for RoR, and why you'd want to read this excellent book. is someone like me who comes from a Perl background and likes the speed and brevity of it compared to Java, but who also has done some Java programming with Servlets and JSP and sees the value of a predictable config files, directory structure, exposure to server environment, session management, etc. In other words at this beginning stage it seems like a wonderful combination of some of the strengths of Perl and Java. I'm not deep enough into it yet to see how it's error checking compares to the robust error-checking of Java and "what do you mean by error-checking" of perl.
I've never paid any attention to "Agile" programming but from what I've read of this book it makes a lot of sense for some projects and ROR seems like a great way to do it.