We have 2 e-ink Kindles, and a kindle Fire and truthfully, have not bought one paper book since I have become "kindleized". Only problem i have occassionally had, is a few books (even some "best sellers") are not always converted to kindle format correctly, but has been only a minor problem - more prevalent on older books that were converted using OCR software and not proofread well. But if you think about it, all of us have booght paper books too that have had flaws as well, so this is no different.
I like the fact also that I can use my Kindle application on my PCs, as well as my Windows Phone, to read a book, if I happen to forget my Kindle.
But I would never switch away from using e-readers now, the only non-kindle format media I have bought at all, is a few magazines.
I do have a gripe that not all of the old books I want to re-read are yet available in Kindle format, but that seems to be happening (too slowly from me) - for example, not all of James Michner's books, and other great authors, have all their books available in ereader format.
I have not tried any of the other formats since i use Amazon.com so much for other things too (like movie downloads, MP3's etc, Kindle free reading library), and been happy with their service.