Preamble: The problem of writing a defense of a certain thing X is that, most people interpret that as an attack to potential alternatives. This is not how this post should be read. There is no One Single Solution. My defense of Groovy is based on a set of assumptions that do not hold true for many people. In fact, they do not even hold true for myself. Different people and different development problems entail different solutions.
My main assumption in defense of Groovy is that you are, a Java person. Java is your day to day programming language and you are comfortable with that. Though you are comfortable with that you want to try something different: maybe you want to try a scripting language, maybe you do not like too much boilerplate code.
Why Groovy? Because it gives you a lot of goodies with essentially no learning curve. An illustrative example: I’ve spent the whole morning thinking that I was editing a Java source file, but I was indeed working on a Groovy file. You see, Groovy not being a superset of Java ends up being almost that: Code in Java is, in most cases already code in Groovy. So, if you know Java, you can write Groovy. You will not write the best idiomatic Groovy, you will not gain any of Groovy’s goodies (and you will pay a performance penalty, BTW). But this is an amazing head-start if you want to go in the direction of higher-level languages (I would argue that it is even smoother than C from C++ as the paradigm does not change from Java to Groovy, it is OO to OO). If you go the Jython way then you have to learn a new language. If you go Scala or Clojure then you have to learn a whole new paradigm (and deal with the impedance of imperative semantics in typical Java libs against standard functional semantics).
For little to no cost you have now many goodies associated with scripting languages (dynamic, low boiler-plate coding, DSLs, better meta-programming, …). In some cases Groovy even out-competes supposedly more elegant languages (are Scala meta-programming facilities still as bad as in the past?).
Another interesting advantage of Groovy is that, if you want to revert back to Java then it is much easier. Why would you want to do that? Well, for performance reasons. In a Groovy application that I have, a small part of the code is extremely intensive, so I had to rewrite it in Java. This revealed to be a trivial exercise (similar syntax, similar semantics).
Again, let me stress out this: your requirements and your personal path are fundamental in any decision you take. There is no true language (OK, Prolog…. 
). Different people, different approaches. All I am saying is: if your background is strongly grounded on Java and you feel comfortable with that, then Groovy is probably the way to go.
Disclaimer: While I have a couple of applications made in Groovy, most of my scripting efforts in the JVM world involve Jython (another fine language implementation, appropriate in a different set of circumstances) and I also believe, that from a declarative and highly expressive language point of view, what is being done with Clojure certainly deserves mention.
