Luke Palmer went from Perl 5 -> Perl 6 -> Haskell. I know a number of people who code heavily in both Perl and Javascript (myself included). I'm also teaching myself Erlang right now.
Ovid wrote the Prolog module on CPAN, which is a similar braintwist.
My criteria for good software:
- Does it work?
- Can someone else come in, make a change, and be reasonably certain no bugs were introduced?