We switched to Passenger on SportSpyder.com from a mongrel + mongrelcluster + apache setup about a month ago. The installation was dead simple and it seems to work great. We do have a memory leak some where and see RAM usage go up to the point where we restart passenger once a day but its nothing that's not manageable. This is with about 150,000 page hits a day (including bots).

Also I want to say Zed was very helpful debugging problems when we were using Mongrel in the early days and I appreciate his contributions.