I've been using the web client exclusively. I thought it was the right thing to do and it WAS until HTML rendering of HTML e-mails went south. Viewing HTML e-mails is such a basic and critical feature of e-mail that I'm having trouble believing there is an issue. My hosted users are about to riot.
I've been a solid user and proponent of Outlook since Outlook 98 came out. I don't know how much longer my users and myself are going to be able to hold out before ditching the webmail client and going back to Outlook/Outlook Express.
After I switched to gmail for my mail email account, I've been using exclusively web-based mail. I've redirected all the rest of my email accounts to my home Zimbra server, and we've ditched Tbird at work for ZCS as well.
The only email client I still use is my phone, and that uses Zimbra Mobile. :)