What do you use instead of Calibre? I haven't found a similar tool that is as useful, is free, runs on Linux, etc.

I find Calibre is not a tool to "enjoy" spending time on, but simply a tool to solve something specific so that I can then spend time on my Kindle.

There are other tools where I want to enjoy the time I spend using them (say, text editors, paint programs, IDEs) but Calibre is not one of them.