It depends what kind of game you're creating. For the kind of game we built it was a huge win. Some of the features were deployed in days.
Ruby might be slow, but in my experience as long as you give it a lot of RAM to play with it can give you great performance (we have over 200k registered players). Of course, we also cache data heavily using memcached.
As long as I have you on the line, do you have any nuggets of wisdom for front end development? I was very impressed that the core gameplay appeared to be just Javascript, AJAX, and lots and lots of bubblegum.
