Pencil Project – Any experience/views?

I’ve been made aware of a Firefox extension for diagraming/GUI prototyping called Pencil. I’m going to take a look at it, to see how it compares to old stagers like Visio, and newer whizz-bang options like Axure, but I was wondering if anyone else has any experience or opinions on this tool?

http://www.evolus.vn/Pencil/