As most have said, I prefer PDFs for sourcebooks, TROs, game manuals, that sort of things. For novels, I prefer EPUB. I use Calibre to manage my ebooks, so getting it on a Kindle for me is as easy as one click to convert it to AZW3 (which you really should be using instead of MOBI these days). MOBI is an old and limited format, and I wouldn't want to purchase anything where that's the only option provided.