It's quite ugly among Mac apps, since it's a Qt app. I usually avoid non-Mac apps, but calibre does what I need it to do, and there is no better option.

The feature set is truly impressive. Hard to believe it's essentially the work of one developer. I don't mean to criticize when I say it's ugly: I'm a back-end developer primarily, and I don't have any idea how to cram as many features as it has in any interface without it becoming ugly.