For years I have been using Outlook for email. It was the first program that allowed full feature email that allowed you to easily setup the half dozen email boxes I use. At the time Netscape (predecessor to Firefox and Thunderbird) offered a full feature email package but you had to close out and open each mail box separately. Netscape added multiple mail boxes several years ago but I was committed to Outlook.
The combination of Outlook 2007 and Vista was the straw that broke the camels back. The combination was frustrating to use. Outlook 2007 is quite bloated and slow, combine the with Vista’s trick of getting the pin wheel for several minutes for no appearant reason it tried the nerves.
The other thing that happened was the open source phenomenon. Plugins are available to connect with other open source resources such as Google. Thunderbird has a calendar plugin called lightning. There is another plugin that allows Lightning to use your Google calendar. Originally I used this to allow Outlook to sync with my Google calendar and re-sync with my windows mobile phone. Now I had a calendar that was on my desktop, phone and the web where I can access this from anywhere I can use a web browser. When I switched to a G1 this is even cleaner, it’s calendar is synced with my google calendar which is also my thunderbird calendar. If I am away from my desk I can add something to my calendar on my phone or using the web.
I switched 6 months ago, there were some bumps but I stuck with it. Now I am totally comfortable in Thunderbird. There is still some frustrating waits, but I attribute this to Vista and switching to more IMAP mail and less pop3.
Thunderbird is multi-platform. I can use Thunderbird on a Mac, in Linux and windows. They all work the same. My goal is to someday be running OSx, Windows, and Linux all at once on virtual machines and switch back and forth at will.
If you use Exchange for email you should use Outlook, but if you do not use Exchange feel free to find the program that works best for you.