I’ve not had luck with EPUB for my Kindle, but MOBIs seem to work just fine.
This seems to be true for all of my friends as well. Maybe it has something to do with the formatting? I’m going to keep trying though because I love downloading free books on my Kindle.
MOBIs are a device exclusive format. They’re primarily intended for Amazon’s Kindle line of hand-held tablet computers that use an operating system developed based on Linux and Google’s Android mobile operating system and are geared specifically towards displaying digital books, newspapers, magazines, textbooks, and other digital publications. For some devices (such as older Kindles) they can be side loaded from a computer if desired.