JRuby 0.9.2 was released a few weeks ago but I just got around to trying it out this morning. My favorite feature is the "JRuby IRB Console". An easy way to experiment with JRuby is to simply to download the console JAR file that conveniently contains the JRuby run time system, has a "tab auto complete", and has readline support. If you have not given JRuby a try yet, download the IRB Console, and follow through the tutorials. Good stuff!
JRuby performance problems:
I love programming in Ruby but find myself working a lot in Common Lisp instead for vastly better runtime performance. JRuby performance is very poor at this time (much slower than the standard Ruby system, which is itself slow), but with support from Sun and some development time, performance and Java platform integration will improve.