Have been using it for years and I can't see what's not to like really. (Except for this recent bug, that admittedly got fixed quickly.) It's fine and the almost only decent, standalone email client out there that is not Outlook.
The only thing I find a bit annoying is that despite years of evolution, it still needs some extensions for basic things such as re-ordering accounts and folders, which to me looks like a very basic feature to support. But hey, the extension works...