I've been very happy with Planet Argon for hosting UrbanDrinks.com so far. There has been zero downtime (that wasn't my fault) and the technical support has been great. Best of all, I'm able to try out different web server setups for the application instead of the standard Apache/FastCGI combo. I've been using Mongrel for the HTTP server (using mongrel_cluster) and nginx as a proxy/load-balancer. So far it's been rock solid.
We did have a few cases where the mongrel and nginx processes were killed, but it turned out to be due to a memory usage issue in RMagick, which was easy to fix with a couple of calls to GC.start in the graph-generation loops.
Anyway, I can highly recommend using Mongrel and nginx for rails apps.. it's much better than dealing with FastCGI processes.