I unpacked my Fire last night, put the first PDF on it 5 minutes ago.
The reader seems fine. It is easy to adjust the font size with the squeeze and spread gestures; it's easy to page through the doc. As expected, portrait and landscape modes switch simply by reorienting the device.
The Fire's screen has an excellent resolution.
The reader does not adjust lines to fit the width of the screen, but I believe doing so would require a format other than PDF (or a new PDF).