We've been using FC for a number of years now. I've been beta testing this new client. It's adding tons of requested features.

I'm always surprised that this product does not get more press on the Mac Platform. It's the only solution I've found that provides mail services, conference groups, group; scheduling, and integrated address book in a cross platform environment. It even has IM and voice services, but I'm not all that familiar with them. Given Microsoft's poor support of the Mac platform, First Class is a nice option. It has servers on OSX, and Win (and soon Linux). Native clients for these platforms as well.

The server is very stable and needs much less maintenance than an exchange server.

Note: the interface definitely take some getting use to. But overall it works very well.