And the reason is not only price reliability etc. BUT they have added the same system they use for physical music sales to physical book sales.
Buy a CD from amazon you get a digital copy uploaded for free to your amazon cloud which is hooked directly into your Kindle.
And now they are starting to add the same for books though it seems most digital copies of books will cost you 99 cents after you have paid for the physical copy.
Also kindle will work with several different book formats without adding any apps.
Also you can have multiple kindles on one account and they all share the media bought for that account. So you don't have to buy multiple copies of books etc. Our family has 3 of them and we never have problems they work well and I would be lost without mine.
Kindle++ it is. Loves to read, loves music and movies. He despises Google anything so just a straight android tablet (like my bloatware Samsung Galaxy) would have not been good. iPad was way out of my price range. Add Prime to the mix and he should be happy...