Let me start by saying that most benchmarks should be taken with a grain of salt, so please do the same here.
I work on Project Laika, and for our deployments we are looking to switch to JRuby. We already have java code hanging around, and it looks like we will need some more soon (I'm thinking it will be easier to deal with SOAP web services in java and calling the classes from JRuby). I wanted to run some numbers to make sure that our performance wouldn't fall through the floor.
I decided to pit Mongrel 1.1.5 running on MRI 1.8.6 against Glassfish v3 Prelude 1 and JRuby 1.1.5 on Java 5. I'm running Rails 2.0.2 in both setups (I know we are behind the times). I ran them both on OS X 10.5.5 and had the Rails apps hit the same MySQL database.
I used ab to grab the numbers. I had it hit the site 1000 times with 10 concurrent requests. I hacked the Laika app slightly so that you didn't have to log in.
I wanted to get a feel for how the app would perform, so I did two simple tests: dynamic content from a typical page and static content. The dynamic content was the patient template library in Laika which contains code that you'd expect in a Rails app: AR pulling info from the DB and putting it into ERB templates. I also pulled down the Rails 404 page to get a feel for serving static content. This is probably less meaningful, as you'll probably have Apache or Nginx serve up your static stuff.
Glassfish, hands down. The results are the average number of milliseconds it took to serve each request.
It beat Mongrel in both static and dynamic content easily. Glassfish v3 also makes it ridiculously easy to deploy Rails apps. You can use the Glassfish gem, and serve up your app with a single command. I installed the Glassfish server, so I could run JEE apps alongside my Rails stuff, there is a single command where you point the app server to the root directory of your app and you're done.
With Rails 2.2 now out and offering thread safety, and JRuby being the only interperter that can take advantage of it... Glassfish and JRuby are really worth checking out.