Functional Programming is old. Ancient. Like right after FORTRAN. But it's the future. Trust me, I'm named after the dude who knew about Jesus 700 years early.
If you're looking for something that will punish your brain and force you to grok the functional world, check out Haskell (http://haskell.org/haskellwiki/Haskell). Some people prefer Ocaml to Haskell.