I assumed this was going to discuss all aspects of Outlook. If all you're using Outlook for is e-mail you're not "wedded" to it; hundreds of programs can give you an identical interface. I liked outlook because of the excellent integration of all its PIM aspects: calendar, tasks, that sort of thing. I currently use Thunderbird with Lightning for the calendar and tasks, but I still think Outlook was best at the overall PIM experience
I use Outlook for the address book (and the calendar to a lesser extent), for the reason that pretty much every application can import from/export to Outlook. No other PIM comes remotely close. It's the only reasonable way to keep my phone, my PDA, my website, etc in sync.
