I have had the opportunity to work with Groovy over the last few weeks, and I though I would share my main conclusion: it is vastly better than Java. Killer features ? Yeah, a few:
Generics ? Better iterator integration ? Yeah, not much worth mentioning.
I also learned how much I got stuck in the Java view of programming. It actually took some time to readjust to some of the Groovy features. The advantages of closures, for instance, are many. But it required conscious effort to apply them when I started in Groovy. So here's the biggest reason every Java developer should be playing with Groovy: because it can make you a better programmer with better reflexes.