JavaWorld has published an article by John Zukowski that looks at the evolution of the Java Collections Framework and alternatives, including new collections-related JSRs, Jakarta Commons Collections, and the Java Generic Library.
If you want to use abstract data types in your programs, the first thing you must learn is the core Collections Framework. Everything else is built upon it, including the proposed JSRs.
Once you learn the basic collections classes, you can branch out to other areas. While you can play with the generic-types support now, you must wait for its final form and the concurrency libraries in J2SE's next version. If you find the Jakarta Commons Collections helpful, by all means, use it. You can avoid creating a commonly used structure like a MultiMap. While a first-year Computer Science student can handle such a task, the fact that it is already created and debugged is beneficial. The Apache license shouldn't be an issue for most people.
Other than generics, I myself haven't ever wanted much more than I already have in java.util, such as ArrayList, HashMap, and HashSet. On the other hand, I did have to implement a Set and List with consistent serialized forms for the ServiceUI API. I'm curious what kind of situations have caused people to look beyond java.util for more collections. How often has that happened to you?
I am anxiously awaiting generics being added to Java. If you have a need for very high performance collection classes check out http://trove4j.sourceforge.net/
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