I've used Groovy extensively for 4 years or so now. A product we used that ran on JBoss decided to use it as the plugin language to extend / use internal APIs. I really pity Java people who don't know or haven't used Groovy yet. It's like embedding Lua into your C app. Why people still write the bulk of their code in pure Java is beyond me.

The other day I learned that Solr supports Groovy as an embedded language to write extensions in. It's amazing.