[pwk]

...is Phusion essentially just a slightly better FastCGI?

Well, it depends on whether you're looking at it from an academic or practical standpoint. AFAIK the technologies are not entirely dissimilar, as you suggest, but there are notable differences once you actually put them into use. I've used both for running rails apps, and a few things stand out for me:

1. Passenger is much easier to set up: it took me about 5 minutes to get my first Passenger based rails app up and running. The last time I set up a rails app w/FastCGI (admittedly a few years ago now) it took me much longer. This comes down to simpler configuration combined with better documentation. Subsequent application deployment is also easier (no manual server restarts required). This is worth a lot in practical terms, even if not flashy.

2. Passenger seems more stable. I never had quite the problems with FastCGI that others appear to have had, and I've only been using Passenger for a few weeks now, but there's a reason people used FastCGI less often once Mongrel became available.

3. Oh, and there's some neat stuff too: automatic spawning and pruning of application instances in response to demand, etc.

[sunkencity]

Yeah, the integrated monitoring in passenger really simplifies rails deployment a lot. It was always very tricky to shepherd the mongrels.