I'm learning Haskell right now. It's the most recent language I've decided to pick up since R back in 2006. Despite dabbling in Lisp about a decade ago, and the functional flavoring of R, it's been a pretty difficult ride. However, it's definitely teaching me to think about writing programs in a very different way.

I can attest to the painfulness of the IO monad. I still haven't gotten it fully. The error messages are also fairly cryptic. The first program I wrote was about 15 lines and it took me about 4 hours of time. But it worked flawlessly and efficiently. The difficulties notwithstanding, this is probably the most fun I've had learning a language ever.