I noticed very similar effects to what are reported here: after moving from Mongrel to Passenger, my memory footprint went WAY down. Leaks went from "holy crap, what am I gonna do??" to "site is working, let's drink beer."

Passenger* is just superb. I kicked Phusion a few bucks, and if you are as pleased as I am then I hope you do too.

* at least on Apache. Passenger on Nginx would crash for me after about a week, so I went back to mongrel_cluster on my non-Apache setup.