We a small technology integrator, currently using Outlook/Exchange via AppRiver's hosted service. AppRiver is absolutely great and a very good value. Excellent support, and the stuff just works. Very reliable. Top notch organization.
I want to move us off of Exchange for a variety of reasons, and want to find the AppRiver equivalent of the Zimbra world. There does not appear to be much customer level feedback out there regarding various experiences with Zimbra hosting providers. I've seen some sketchy feedback regarding 01.com; however, because there aren't that many supporting threads on that, it's hard to read how "credible" the experience of one or two customers is.
Reliability and support responsiveness are probably #1 concerns for a mission critical app like Zimbra. App performance in a hosted environment would be #2 for me. Cost/features would be last.
Would appreciate if people could chime in on their hosting experiences.