Another Webby user here. I forked the repo when I learned it was no longer maintained and I got tired of patching string.rb with "#encoding: UTF-8" whenever I was installing the gem someplace. :)
I tried something else at some point; I forget what, but it was more complicated, so I stuck with Webby.
Extending it is just easy. I've been using it to create mobi and epub files from Textile, as well as plain old Web sites. I think I use it for pretty much every site I have that isn't hitting a database, and even with those I probably have a static section built using Webby.