There was a heated discussion on Slashdot regarding OpenOffice.org including Mozilla Thunderbird with the Lighting plugin in OpenOffice.org 3.0. They are looking to compete with Microsoft Outlook with their own PIM. As a user of both Thunderbird and Outlook user, I can say for certain that unless Microsoft opens up the proprietary protocols that Exchange uses with Outlook, this will never come close to replacing Outlook. It might make for a nice PIM, but it will not make a suitable Outlook replacement.
I have used Lightning, and everytime I tried loading it, it would cause Thunderbird to crash on me. It was so annoying that I gave up and stopped using it. I was looking to do just this and replace Outlook with Thunderbird as I do prefer free and open source software. Unfortunately, it wasn’t to be. I will admit that this was using Thunderbird 1.5 about a year ago. The problems I experienced might have been resolved by now. However, Mozilla seems to be dropping Thunderbird to concentrate solely on Firefox. It leaves me to worry that Thunderbird might get lost and rarely updated. I am also not completely happy that the only way to turn Thunderbird into a full featured PIM is through plugins and extensions.
To fully explain myself, there are features of Outlook when using Exchange that Thunderbird just won’t be able to do, such as syncing the calendar and contact list with the server so that you have all entries automatically in Outlook and on Outlook Web Access. It won’t allow for sharing of calendars or contacts unless the user sends the event or contact to others. I love Thunderbird, but the functionality I get from Outlook when using Exchange is too great to switch back to Thunderbird, at least for work. For home, I just use Gmail and occasionally Thunderbird (though I rarely open Thunderbird).
Flame away, but nothing will sway me on this one. The only arguments I saw on Slashdot about the possibility of this being possible were those people who either did not use Exchange or did not use it for everything that it offered. Yes, I am saying their opinion didn’t matter because they didn’t have all their facts right.