If you are a Java programmer and you’re interested in Ruby, you should read this presentation about 10 Things Every Java Programmer Should Know About Ruby.
This blog entry also has some nice Ruby information for Java developers.
About the most dangerous thing about Ruby is how more elegant and less verbose it is. And this is not only reflected in the core language, but also in the various Ruby frameworks and extensions (such as Ruby on Rails) and the developer community that has adopted these philosophies. Even though I consider myself pretty much a Java expert but only a beginner at Ruby, I already feel more productive solving many problems in Ruby than in Java.
You may find yourself swearing at the verboseness and clumsiness of Java once you’ve done some Ruby programming. Don’t get me wrong - Java is a great language and I enjoy programming in Java, although I guess this is more because I am very familiar with Java and its various libraries, including excellent open source software and IDE support. But it is definitely well suited (and proven) for enterprise software, whereas Ruby still needs to prove itself in this area.