I'm the principal author of one of these example apps (Insoshi, a social networking platform developed as part of YC Winter '08). It's a bit out of date (running on Rails 2.2), but still quite decent. I'm currently working on a Rails tutorial book that will include a fully up-to-date and (I hope) high-quality sample Rails app; keep an eye on http://www.railstutorial.org/ for more details. You should follow the project on Twitter at http://twitter.com/railstutorial for all the latest news. Note: some major announcements are coming soon. :-)
That being said, I recently spent some time reading through the source of both Gemcutter.org and RailsDevelopment.com and found both to be very educational to me personally:
Depending on the audience, maybe. I've not looked at the code, but I know it has some very specific design assumptions and a number of weird corner cases baked in which might not make it a good general-purpose example.
