I use fluxbox but that doesn't prevent using gnome apps; my main issue with them is that god-awful look they all assumed overnight a few years ago, without the title bar and the like (I think to match the Ubuntu tendency at the time / trying to emulate the fucked-up universal touch interface thing Microsoft tried to introduce at the time ?), from gedit to, indeed, evolution. I really loathe it. And thunderbird kept a classic look (firefox didn't, which means regular css tweaking to achieve the same result).

Also thunderbird supports calendars and webdav/webcal sync with plugins (though perhaps evolution does as well now, I haven't checked).