Just finished reading my first eBook, Zendegi, on the Kindle app on iPad.
I was expecting it to be a little tiring reading on the iPad, but it wasn't my eyes that gave me any problems. The iPad screen is totally fine for reading in my opinion. I'm not even sure I would find e-ink that much better now. I found that once I adjusted the brightness of iPad to suit the conditions, it was great. Kindle has an extra slider for in-app brightness control as well, which is handy.
No, the biggest issue I had was the weight of the iPad, it's 600g or so, and you need two hands to hold it, so I guess it is like reading a decent sized hardback. If you are holding it up in front of you, it can get tiring. However, if you have it resting on your lap, or a pillow on your lap, it is actually more comfortable because turning pages takes only the slightest, smallest thumb jab, or swipe, rather than the wristy physical page turn of a real book.
I was using the Kindle app rather than iBooks, because iBooks has zero content in Aus, and the Kindle app is really nice as well. Touch a word, and you get instant in-page dictionary lookup, and if it is not a dictionary word, there is a link to Google to start a more detailed hunt for meaning. The page turning animation is not as cute, but I think that might get tiring after a few hundred pages, and the Kindle slide is really good.
One difficulty I had was that you don't have an immediate sense of what page you are up to, and in fact the pages aren't even numbered, because it depends on what size font you use. Instead, you can get a percentage done in the menu mode, but a tap away. It just feels a bit wrong not knowing how thick the book, or how many pages you have read.
The other thing I will miss is that now I can't display a physical reminder of the read on a bookshelf in the lounge room. Maybe I could make a cardboard box of the right dimensions and print out the cover and spine to paste on it.
Given it is cheaper, and in many ways better, I don't think I'll be going back to physical books unless there are supply issues, but that seems to be getting better all the time.