I recently made a full website redesign using only F/L/OSS tools and tested pencil, wich I didn’t knew at the time and Inkscape, wich I am a long ter user but only for printing/illustration porpouses.
I found Pencil quite clumsy and there were a lot of small papercuts with simple things like alignment, element seelcting, font-size, layers… Also missed some easier way to use custom graphics, I did a quick research about, saw that it was no so straight-forward and gave up.
For user-testing porpouses I preferred Inkscape because I could show the protoype with a more refined look.
Links to the Pencil screens and Inkscape Screens. (both in .png)