Hi Dave.
This page should answer all your questions, I think. But very short summary--there are lots of us using Zimbra Open Source who are using the web client (called the Ajax client on the table), and also connecting to Zimbra using either the Zimbra Desktop or one of several IMAP clients such as Thunderbird or Outlook Express. With the Thunderbird Lightning extension we even have calendaring, though it's not perfect yet. We get spam filtering, virus filtering, and centralized monitoring.
I'm sure I'm missing somebody else's favorite features, but these are the ones I use.
Dan