I’m writing this post from an iPad, I’ll write a longer comment about the sustainability problems and possibilities of this device. But I wanted to make a short entry on my experience of using it as an ereader (electronic reading is one of my interests).
Positive:

• The screen is gorgeous, the colors crisp and the viewing angle is good. The size is perfect for documents.
• Interactivity allows to do things as searching an unknown word in the dictionary, or copy it to your notes program.
• iBook program and store are neat and easy to use.

Negative:

• The screen is a normal light-emitting and it’s too tiring for comfortable long time reading
• It’s quite heavy and difficult to hold in one hand, for instance for reading in bed.
• Having internet allows multitasking and makes focusing in reading more difficult.

So after some use it confirms the expectations I had: the iPad is perfect for reading magazines in color and articles and documents. That together with the text editor and email make it a killer education/work device. But for reading books, the e-ink based devices as the Sony or the kindle are still the best option.