REBOL had some novel ideas, and some shortcuts for some particular types of development making for strikingly concise code in some specific cases, and overall a nice design. I think it's neat that there's an OSS implementation, but I have so many other things on my todo list that I've never done more than give it a glance. I just don't see how I'd make productive use of it in the kinds of projects I do (given interop requirements I have).