I use the mail app that is part of Mozilla. It handles multiple accounts well and you don't have to start up two programs to do mail and web browsing. Another big plus is that Mozilla has versions for OS X, OS 9, and Windows. I can switch from any flavour of Mac and then to a PC and have a familiar interface and setup. So far I have been really happy with the latest stable version of Mozilla 1.2.1
I use mail.app it is simple and clean and does everthing I want. Now with Jag I there is and awesome address book and iCal, so I can use iSync to sync with my iPod and have everything I need at my finger tips.
I should also mention the I, like others here just don'y like Microcrap, I am mean Microsoft. So my opinion may be a little biased.
I just believe a company that tries to excel in everything, will end up doing nothing well.
Mail.app for me as well. Sometimes I miss the integration that Entourage X offers, but then I just remind myself that it is one more step in weaning myself off Microsoft apps. Apple seems headed in the right direction with some of the recent steps they have taken in offering integration amongst some of the iLife apps, so perhaps we will see more seemless interaction between Mail, Address Book, and iCal in the near future.
I have been using Outlook in one way or another for so long I couldn't imagine trying to export my contacts, calendar events and mail into anything else. Especially something that is so young an app. All I need at this point would be be to move over and then apple stop supporting the app.
I am very happy with Entourage and have no real viable reason to change.