The developer has been helpful enough on the couple of times I have filed a bug.
I'm not sure why people hate the UI. The configuration UI is pretty bad, but I rarely go need to go there and the main screen works perfectly for me.
I didn't mention that I also don't need to use Calibre as I can load my epubs onto the device via wireless connection.
I also don't use Amazon's ereader software so that I'm able to read epubs without converting them and also having an UI interface on the Kindle that suits my habits better than the official one.
For these reasons I don't have a need for Calibre in the first place.