I use my smartphone for both Kindle & Nook books. Also MS Reader and Adobe formats. It is smaller than the readers so the screen size might not be the best but it's one less thing I have to carry around or find another outlet for the charger.
I understand the craze about the e-book readers but I was all ready reading e-books on my smart phone before the first Kindle hit the market so didn't really understand the fuss.
The good thing for me is that with everyone & their brother now owning a Kindle, Nook or Ipad--more & more books are available for download and I can read almost any book I want on my phone and can get the best price since it can handle so many different formats.