A more recent commercial site that is using Kepler (via the Sputnik wiki/cms) is the site for the "World of Warcraft Programming" book, created by the book's author. I have updated http://www.keplerproject.org/en/Uses (the current Kepler homepage itself uses Sputnik, too) to reflect that, and link to the Sputnik sightings page.

The big con is that the included batteries have very low wattage-hour. :-) A lot of things that you can take for granted on the big Ruby/Python frameworks you will have to build yourself.

The upside is that once you get the ball rolling it's very easy to deploy, and will run without issues on the most stingy shared hosting (Lua is very CGI-friendly). It can run on any server that supports CGI or FastCGI, and also includes a Lua webserver (single-threaded only at the time, though).