have you? How does it run?

Fast. I initially had some set-up issues making sure the MySQL jars were found and tweaking Glassfish settings.

Also, development of Ruby + Glassfish almost always meant Rails + Glassfish. I prefer Ramaze, and had to hack a few things (in warbler, for example, and the glassfish3 gem) to get around some things that that assumed you were using Rails.

If you are using one of the anointed frameworks (i.e. Rails or Sinatra) you're probably OK. If you are coloring outside the lines you may have issues. But I've not looked into it much since I got my site running (though now I'm just running it as a rackup app through jruby)

At this point though I'm looking at using Haskell when speed is a concern. But that's a whole other thread ...