I've been toying with Factor for about a week. It's been an enjoyable experience, as both the language itself and the included development environment are extremely well designed.
Having never played with Forth-like languages before, I'm finding writing even the simplest algorithms strains my mind -- in a good way, though. It's mind expanding in the same way Lisp was the first time.
I doubt that Factor is the next big thing, but I think that for smart people it's one of the most fascinating things around right now. Zed Shaw, for example, is a big fan: