It's quick to convert but I've had varying results going from epub -> mobi, especially with calibre. What's worked for me the best has definitely been kindlegen but it's still not perfect.
In the Calibre settings, did you specify the exact version of the Kindle you have as the output device? With that set correctly, and not just left as the default, it has been years now since I had any formatting issues going from epub > mobi/azw to read books on my Kindle Paperwhite.
