I use outlook. In fact, I just measured the load-up time for Outlook and gmail.
Outlook: 12 seconds
Firefox+Gmail: 21 seconds.
Finding and opening messages is much easier in Outlook. I can use tab and arrow keys to navigate much easier than I can with gmail, and messages load much more quickly. Since it's an Exchange server that maintains a constant connection, I get email from anyone on the network within a few seconds, and a notification in the taskbar that I've got a new message. Mail filters are not difficult to set up, and arranging messages is a drag-and-drop affair.
I'm using Outlook less these days, as my co-workers tend to use IRC for most incidental communication and email for technical notifications and documentation and such. But at my last company most professionals, managers, and executives lived in Outlook.
There are a few things that really bug me about Outlook: No support for tabbed browsing of messages and a buggy, poorly documented scripting engine. Practically speaking, Outlook requires more IT support than gmail.