Evolution looks neat but for me a huge problem is the lack of crossplatform-ness. I use windows as my main desktop OS simply because I like to play a lot of games in my free time, but at the same time I use Linux for development (at work and at home), thus it's quite important that I can use certain programms on both machines.

Thunderbird seems like the only cross-platform open-source mailcient that that is at least somewhat alive.