I only use the Amazon site. I have never had any issues finding pretty much anything I needed there, and love Whispersync, so, I have stuck with them. I probably have 250 books or so, and love that I have a library at my fingertips no matter where I am!
I think that Amazon spent a lot of money developing the e-book segment of their business, and the few books I have not been able to find would not be anywhere else, so never tried any other e-book "vendors". Although, I was given a Barnes and Noble gift card, and since I have pretty much become an e-reader, decided to download the Nook app. But I like the convenience of having everything in one place so much, that I have found that I haven't used that one except for one "experimental" book, and that one was equivalent to my Amazon books with the exception of price. As with iBooks, the Nook books were also more expensive than the Amazon Kindle books.
Also, I have the Kindle app on all my devices (iPad, iPhone, and Macbook Air), so what's not to to like, huh?
Hey, just out of curiosity, is your nickname in any way related to "Pat" as in Patriots, as in New England Patriots? As you can see, I'm a New Englander, so, of course, when we see "Pat" that's what we immediately think of....hahaha.....or are you just PatMan, like in PatMan?
GB