I disagree with his statement about the Kindle Fire not having built-in support for .mobi files. I am currently reading Land of Lisp in .mobi format on my Kindle fire with no problems. The one quirk I did notice is even though I uploaded the .mobi file to the books directory, the book shows up in the Docs view. It seems only books purchased directly through the Kindle store are allowed to show up in the Books view.
I generally like my Kindle Fire but I do find some of the on screen buttons require multiple presses to respond, particularly the back button. Hopefully this will be fixed in a software update.