As the GRails hit the version 1.0 last week, there are so much articles about Groovy began to appear, more and more.
And this is the only beginning, case there are so much java developers tired of java and I predict a big hype around those stuffs ;). I had some experience with Groovy and GRails several monthes ago, when GRails was at version 0.5 and 0.6, that's a quite complex feeling, and this is about them.
Groovy, byitself is a tiny little ray of light in the Java world. Seriously. I'm not a big Java guru, but in my opinion, Groovy is the best damn thing happened with Java. You probably won't have so much beauty and easy feeling like with Ruby and definitely you won't have this cool-hacker feeling like with Python, but anyway Groovy is a very, very good language. It's a bit strange experience at the beginning, you try use it like Ruby and actually mostly can't, then you usually use the Java native libraries and all the time falling back to the Java coding approach. But after some tries and fails, after some readings, you get the idea of Groovy and it becomes more or less handy.
There are some good ideas in Groovy, like the how do they define everything, the authors were definitely inspired by a functional programming practicies, and as result you may do your lovely FP-paradigm with Groovy very much, there are all those nice stuffs like functions and carrying and so one. But then you always can fallback to more usual OOP stuffs. And definitely you may allow yourself all those metaprogramming goods you always been missing in Java :)
GRails, as I mentioned, I tried it when it was at version 0.5 and 0.6, and later I've looked through the version 1.0-rc. That was a quite, quite raw stuff those time. The behaviour was not much stable, always had some stranges in it, and sure I was not such happy and productive live with the original Rails. I would say it's a rails for java-developers (in the bad meaning of the word).
Heck those guys even don't support their own java-world stuff maven. The domain model constructions, like it usually is in java-world, very abstract and keeps you as much far from database as it possible. (I don't know why, but most Java-developers afraid SQL just as a fire). You don't have normal REST-idelogy support, you do have a doubtfull rest-pluging, but grails by itself don't have anything in it like that. I don't like the project's configuration, include the routes configuration, it was made out of hands. The testing organisation is quite bad. The internalization layer is "as usual in Java",- read rubbish. Very poor (at last after Rails) builtin functionality. And the biggest sin is no documentation. Almost at all. There are couple of useless books about Grails of old versions and some miserable articles and that was all. Almost no correct API documentation, no tips, not triks. And one strange screencast how to install grials with a horrible german accent :)
The only thing I liked in GRails at the time was the tag-libs feature. This is almost the same feature which you have in JSP, but you can define the tag-libs in Groovy, similar to the Rails helpers. And that was implemented quite handy, with namespaces, with normal inner blocks handling, etc. That's what I would like to have in Rails instead of erb.