I would call the Kindle Fire a media device rather than a true e-reader. Sure, you can read a book with the Fire, but you can also "read" with a laptop, desktop, and a 54" Sony Bravia LCD TV.
Those of us who truly read, for hours on end, know that e-ink displays are superior to non e-ink displays for reading. There just is no comparison to a back-lit device.
I haven't bought a physical book since receiving the Kindle as a gift more than two years ago. It completely changed the way I read. Easier on the eyes, portable, convenient...I don't think I will ever buy a physical book again (and I work for a book publisher!).
I like the idea of a media device, but for serious reads, a tablet is no substitute for a dedicated e-ink device.