I only use Gmail. Here's why (responses to Jared's reasons for using Outlook below, first allow me to share my mentality).
I am a confessed obsessive organizer. Yes, I'm one of those people who can't bear to have clutter on their desktops (whether physical or virtual), have an unsatisfied desire to clean out their Inbox and have a general liking for right angles. While I'm not quite Adrian Monk, my tidiness does help me put stuff where I can find it (usually!).
For years I lived in an Outlook-based world. For an organizer like me that means filing my email in folders and subfolders. Then came Gmail and with it Search Don't Sort. Would you believe, I stopped organizing, and started getting some work done. The confidence of knowing that I'll always find what I need, when I need it is a staggering productivity driver.
Of course, I still organize my email - but not by categories or projects. Powered by Andy Mitchell's wonderful GTDInbox, my Gmail has become a the host for Getting Things Done. Perhaps I do perform the GTD steps of process and organize a little more than I should, but hey, I can indulge myself can't I? :-)
Otherwise I don't feel I am missing any of the reasons Jared still uses Outlook:
The Offline Experience: am I ever offline? Perhaps on the plane, and then I catch up on my RSS feeds (Google Reader with Gears).
Tasks: GTDInbox, 'Nuf Said.
Message Flagging: granted I don't have this, but putting an item in the GTD "Waiting On" bucket does the job for me.
Mobile Phone Experience: I may be behind the time but Google's Java app worked great on my old Nokia 6230 and works even better on my new Sony Ericsson. And I can search from the phone! (so it's better than the IMAP sync I could have opted for.)
Loving My Add-Ins: on the add-on front Firefox and Gmail win hands down... between Lifehacker's Better Gmail and who knows how many more add-ons and userscripts, you don't know where to start. (Start with GTDInbox, of course (-: )
Thank goodness I got my company using Google Apps from the get-go. I for one hope to never return to that land of Outlook and Exchange...!