The Thunderbird Mail Project has legs, but not quite wings.
I have used Outlook to manage my correspondence for many years, and over the last 2 years I have enthusiastically and reluctntly converted to Evolution. Up until a few weeks ago I wanted Outlook on my XP and Evo on the good stuff.
"Save messages on server" is a must in this scenario, 2 environments 2 email clients, no option. But that is where the TBird starts soaring, being 'standards compliant' no matter how loose the standard, means that I can share the client files not just the connection. So what?
I have an ant script that makes heavy use of the sync task to keep my laptop, desktop, and home systems in a 'ready for action' state. It is along the lines of Martin's Subversioned Desktop, but without the version control. Almost every project I have been involved with starts with an exchange of very meaningful emails, and being able to access and search these emails is relatively important to the success of the project. In the past I have found myself converting Outlook PST files when necessary, it all depends on what I was doing when I received the pertinient email.
TBird allows me to share a common email folder in both Windows and Linux environments. And its news/rss etc. integration gives you the ability to mix emails with targeted news. I am still a long way from reaching the limits of what it can do to merge and cross polenate ideas between fields.
However, there is no calendar/scheduling function. Mozilla consider this to be a separate project, but they have 2 projects that fit that niche. Unusually, they are letting both run - one version targeted at integration with email and browser technologies and the other designed to work on its own. Microsoft have proved the value of such integrated applications, and I hope that Mozilla can make the right decisions in this area. In the mean time the 2 project complement very nicely the 'de facto' functionality provided.
TBird is nothing special on its own. It is a mixture of familiar common email features; which makes reading the manual almost obsolete. But what is does do is free you from a proprietary email system - you can even grep your emails. To sum it up, Thunderbird is sharing friendly.