I really like my Kindle 3 keyboard. I have found the built-in Linux environment to be a lot of fun, and there is just enough openness so that I feel I could do anything I wanted hack-wise if properly motivated. There is definitely some proprietariness to it, but coming from other platforms like TiVo, iPhone, Mac it's not bad. At least Amazon isn't openly hacker-hostile... yet... ; )
My point is that maybe Android reader has more to offer in gnu open-ness, but I think Kindle is such an excellent product, it wins when using it for it's intended purpose.