A couple of years ago, I started playing with Python, Scala, Ruby and Groovy time to time. The very first thing that really impressed me was their expressiveness.
I really like all of these languages, but here in this post I’d like to emphasize the benefits of Groovy for Java developers since Groovy is the easiest to learn and to make transition to a “Java.next()” language for them.
Groovy is Expressive
Groovy is a very expressive language compared to Java. You can do more by less code. Expressions and syntax are very similar to Java, but they are shorter. Actually, some of them are like shortcuts for corresponding Java expressions.
Flat Learning Curve
It has a flat learning curve. It is very easy to understand the code and easy to write for a Java developer.
You can start with Java style coding in groovy (removing semicolons from java code:) ), and make a smooth transition to groovy style by simply removed redundant keywords, and using shorter (Groovy) version of the expressions.
"A Java developer embarking on a Groovy adventure will always have Java in mind, and will progressively learn Groovy, one feature at a time, becoming more productive and writing more idiomatic Groovy code. " 
Similar to Java and Also Different from Java
Groovy compiles to Java classes and byte code therefore it can use all of Java platform.
Yes, Java developers, it is like Java you know, it is on JVM. You dont need to learn new environment, and new dynamics of a language.
It extends the JDK. You have all libraries and features from Java, and in addition it provides a number of features from languages such as Python and Ruby. Closures, metaprogramming and dynamic typing are just a few examples of those features.
It also supports DSLs, mini-languages aiming to represent constructs for a given domain. It helps to make code easy to read and more maintainable.
Groovy has also nice collection processing features. It eases the creation of collection proving a more natural way. Also,the code is definitely more readable.
Groovy is dynamic and optionally typed language. You can keep types if you want, and also can use dynamic typing. It is nice that you don’t have to worry about typing. I find it very powerful in scripting and writing some quick codes. However, in bigger and enterprise applications you should be careful. It is great that you can write very succinct code but the cost of not knowing what it is being passed back and forth can be sometimes not so nice.
By doing more in smaller peice of code, it really feels more productive. For example, let’s say you have an idea and/or design and you just want to write code to implement it quickly. Instead of dealing with syntax of the language, you can concentrate more on the stuff that you are trying to solve.
It can be used as a scripting language for the Java Platform, and it can be used to have some small hacks on the daily development. It is a nice alternative to shell scripting for developers with a Java background.
Warning: Power comes with responsibility
It is true that it is flexible and powerful language with many new feature such as dynamic typing, closures,etc… It can easily go in wrong direction, and you may end up with unmaintainable and messy code.
Although Groovy is supported by IDEs such as Eclipse, and IntelliJ , it provides less information for IDE since it is dynamic. The compile time checking, code completion, and refactorings are limited compared to Java.
As conclusion, I strongly recommend Java developers to learn Groovy and start using it somewhere, either as scripting language to do some daily small hacks or in some part of your projects. You will really feel more productive and see the benefits.
Even if you don’t use it anywhere, it will give you a perspective and widen your vision.
And also please keep as a general rule in mind that all languages are like tools in your toolbox. Just use the most suitable one, which is not necessarily the most popular one, not necessarily the one you like most and not necessarily the one you know most. In short, as I said in this blog: No language fanatism!
 http://groovy.codehaus.org/ (image)