One case that comes to mind - quite often when I do unit-testing in JRuby, and run a single test from the command-line, the test will take 1sec with JRuby vs al

One case that comes to mind - quite often when I do unit-testing in JRuby, and run a single test from the command-line, the test will take 1sec with JRuby vs almost nothing in regular Ruby. So I find myself running the individual test using MRI Ruby while developing, although the whole test run will be done with JRuby in the end, just because of startup time.