This review is based on 30-days of service, and while that isn't a long time - I've been impressed enough to post. We've been using AppRiver (www.appriver.com) for Hosted Exchange, primarily due to the calendar & task features. The company I work for provides Linux-based web hosting services, and for e-mail that solution has been more than adequate - but it takes a bit more than e-mail alone to stay organized.
Hands down, AppRiver's product is the best we've used for hosted Exchange. We've used several other providers for similar services in the past, and haven't found one that fit well for our needs until now. They've got responsive servers, responsive support, and a reliable product.
Specifically, I was impressed with the following:
1) Support. We've made only a few support requests; but when we have they've been answered quickly and intelligently. In one case, we were answered by a developer who offered (and followed through) to add an additional feature to suit our needs.
2) Control Panel. The "Shoreline" control panel offered by AppRiver is fast, easy to use, and rather intuitive. Setup of users, aliases, and getting applications such as Outlook and ActiveSync set up was simple. No learning curve required.
3) FAST Servers. Other hosted exchange services we've used suffered from tremendous lag; even to the point of causing Outlook to not work. Using AppRiver we haven't seen any latency or slowness, whether we were accessing via Outlook, a handheld, or OWA.
Overall, I can honestly say that I haven't been more pleased with a application service provider, ever.