Ruby Complements Java
The cost of software development and maintenance is usually the largest expense for a company's IT budgetmuch more expensive than servers, Internet connectivity, etc. The use of Ruby can greatly reduce the cost of building and maintaining systems, mostly because programs tend to be a lot shorter (For me, the time spent per line of code is similar for most programming languages I use).
OK, so when should you use Java? I have used the Java platform for building systems for my consulting customers for over 10 years, and I certainly will continue using Java. A well-built, Java-based web application will run foreveror at least until servers fail or have to be rebooted for hardware maintenance. My confidence comes from seeing systems run unattended for months on end with no problems. My advice is to continue using Java on the server side for large systems and to start using Ruby for small utility programs. For my work, I view Java and Ruby as complementary, and not as competitors. Use the best tool for each task.