For me, the big win of Passenger is freeing myself from manual administration of Rails apps. Any performance gain is secondary to the fact that I can drink beer while I rest assured my apps are being served.
My reasoning prior to today's announcement? The convenience of the deploy. The fact that I don't have to dive into Ebb/Thin/Mongrel and figure that out (probably easy for 99% of the people on this thread) means more time to think about other things.
It's also exciting from the perspective that I'm interested in seeing what the Ebb/Thin/Mongrel teams are going to do. Competition is great!