At least the Apple iTunes ecosystem acknowledges my mp3's as music and generally as first class citizens for everything but iCloud.
What Amazon does is childish and annoying.
Calibre can't properly integrate with Amazon's proprietary ecosystem, so you're stuck with 2 separate ways of managing your books, and a are pretty much encouraged to rip out the DRM. (Okay, so I would have done the latter anyway without the encouragement.)
And then you still have a reader that says "na na na na na, those aren't really books!"
Nice reader though.