The idea that the iPad is difficult to read on has been spread mostly by.. surprise surprise... people that haven't actually read on it.
I'm sure some people may not be able to look at the screen for long. They're the same people that can't look at any computer screen for long.
Yes, you can have glare issues on rare occasion. They're usually solved by shifting the iPad slightly.
Most of us stare at screen over 8 hours a day. I can understand if you want to go home and look at something else but honestly, it has yet to bother me.
The reason so few eBooks have been sold is because the selection is weak. The books I want are almost always in the Kindle store and are rarely in iBooks. However I still use my iPad to read those Kindle books.
Additionally, that's 1.5 books a person via iBooks alone, over a 4 week period, that doesn't count the Kindle purchases. That number seems pretty good to me.