I've been using my iPad one for reading every day since pretty much when iBooks came out, about 1.5-2 hours per day. That was fine because I like to read in bed, in the dark, where the Kindle wouldn't work very well. The new iPad is even better as the text is of course much sharper.
Two things that still bother me about it for reading:
1. The black levels are still not great, especially if you use iBooks "Night Theme" where the whole screen is black and the text is white. The "black" is actually a glowing grey, and reducing the screen brightness doesn't help.
2. It's a bit heavy, even if you just balance it on your chest/legs/gut for that long. It's OK would I would prefer a smaller, lighter model. 7"-8" screen and something like .8 lbs. or less would be ideal, even if the battery life was only 6-7 hours.
If I didn't read in the dark all the time, I'd probably get a Kindle, but having to keep a light on and always oriented to the screen is a PITA, so the iPad works OK for now. No eyestrain but it's not the perfect reading experience. However, I do love the syncing of book library and bookmarks over iCloud, and the dictionary and search capabilities are really useful.