Actually I have an iPhone and have used available hacks to allow third party applications to run on it. I have installed the ereader, details here:
http://www.modmyiphone.com/nativeapps/books/
Supports text and html files.
The screen is small but works quite well for reading. Easier on the eyes when you invert the colors, i.e. white text on a black background. I find it easiest to scroll through the pages by scrolling the screen up with my finger. Very nice to have an ultraportable reader.
Still, I find myself wanting to buy a dedicated eReader. The screen is small for long reading sessions, and while very clear, the back lit display does cause some extra eye strain. I am getting closer to allowing myself to spend the $300 for the Sony PRS505. Likely I will not hold out much longer.