I switched from Mongrel Cluster to Passenger two weeks ago (Debian Linux Server). I didn’t look back for a second. Passenger is probably the easiest way to get your new server up and running. Performance and reliability are reasonable too.
Personally, I like to spend my time working on exciting new Rails projects rather than dealing with deployment issues – Passenger enables me to do exactly that. However, Mongrel or something else may still be preferable if you have some kind special requirements (doesn’t apply for most products).