Larry Gilbert

It's big, it's bloated, but so much of the Web depends on it that it would leave a gigantic vacuum were it to disappear.

Rjb is good to have when you want to call some Java code but you can't or don't want to use JRuby. The most serious limitation is lack of support for multithreaded Java code (which I think is a limitation of MRI's design and not Rjb). The only other potential problem is that it uses syntax different than JRuby's, so (a) conversion from JRuby code takes some work, and (b) you have to go through some contortions to make code that's compatible with both Rjb *and* JRuby (Apache Bundlr, for example, has its own compatibility layer for factoring out the differences between the two).