This email program is stuck in some awful interface hell from the days of MacOS X's public beta. I'm not entirely sure what makes it so unbearable, but suffice to say that using it is both confusing and unpleasant, from an interface perspective.
Aside from the interface, however, this email program is lighting quick at IMAP interactions of all sorts, and is probably the bar none best IMAP email client out there, from an under-the-hood perspective.
So, ugly on the outside, sweet well engineered IMAP goodness on the inside.
It would be a KILLER Linux app, but it struggles with the Mac market for these reasons. But hey, give it a try, see what you think.