Like Illinifan, I often use the sepia and turn the brightness up or down based on the ambient light of where I'm reading on my iPad2. It does make it easier on my eyes.
I have a K1 and a K3 and have pre-ordered the Fire. While I like the e-ink of the K1 and 3, I'm hoping the Fire offers a few more features for making Notes and such. (I also have the 1st gen DX, but haven't used it much since getting the iPad. The DX was great for reading research PDFs at the time it first came out, but the Goodreader app on the iPad far surpasses it now.)