It is the Gimp of book management.

Both Calibre and Gimp are great examples of outsider design.

Works amazingly well.

Looks amazingly bad.

When I first used Calibre I thought it was a disorganised mess trying to cram too many features in one app.

Over time I learned to appreciate the vision: it’s not the stereotypical modern streamlined app, it’s the cockpit of a spaceship. It’s AutoCAD for ebooks. I really enjoy that application now, I haven’t even scratched the surface of its featureset.

I heard the author gets very defensive of his design choices: good, I wish we had more opinionated app developers in FOSS.

I don't see why people are complaining about the UI. The calibre codebase is just Python and you can vibe code/customize a nicer UI for it in minutes. It's not like the core changes much. Their metadata resolution and Epub handling and (formerly DRM cracking) are best in class.