This may sound ignorant, but if you're going to use another JVM language, why not use JRuby (if you want to stick with a Smalltalk/Modula-3 derivative), Clojure (if you like Lisp and want STM) or Scala (if you want the static type checking, speed, built in support in IntelliJ)?

This is a serious question: several people I respect (who are smarter than me and thus well capable of learning new languages) are using Groovy for their projects and I am wondering why?