"For all its strengths, however, Groovy suffered from several important drawbacks: the first and most painful was performance. It was slow — even for a dynamic language. The second limitation was that Groovy had no real design philosophy, save for a wide-ranging pragmatism. Any new feature that brightened enough eyes in the community got thrown in. So the language grew organically in ways that were hard to predict, rather than having a clear philosophy and a long-term trajectory. Finally, there was the problem that I believe most cramped its acceptance: limited documentation. There was one "Bible" and a pair of tutorials that rehashed the basics."
I wrote a lot of doco for Groovy over 5 yrs ago, but it has a tendency to break between different versions of Groovy. E.g. this doco http://groovy.codehaus.org/JN2515-Closures I wrote stopped working after Groovy 1.5 because the Groovy despots suddenly decided to disallow try statements without a catch or finally clause for Groovy 1.6. I had to go through all my code and add finally{} throughout it all. I put the root problem down to the despots not taking work on the Groovy language spec (JSR-241) seriously.
Let's look at the main tutorial, Groovy in Action. The first edition came out in 2007. In 2009, they began work on the 2nd ed. IT IS STILL NOT AVAILABLE except as an "early release book" [1]
What mystifies me in all this is that Groovy is a language with a company behind it. How can they not have made more than such a feeble attempt to provide current docs?
[1] http://www.manning.com/koenig2/