Plus, of course, it is really easy to get more books. All I need is WiFi access.
The experience does vary by reader. All of them are usable but some are more mature. Here are my experiences.
This is the most mature product. It automatically orients which ever way you hold it. Page turns only take a short swipe. I can easily hold the tablet in my left hand, balanced on my leg and turn pages with my thumb. The page flip shows as new text replacing the old text from right to left. The app lets me adjust the font size and margins. It has three viewing options - black on white, black on sepia, and white on black. I can read for a while on my tablet then open the book on my phone and the Kindle app will automatically offer to sync the position in the book. I can tap on the screen and see a progress bar at the bottom showing how far I am into the book. the bookshelf view also gives an indication of how long a book is and how far you are into it with a row of dots beside each book.
The Kindle store (Amazon) has a wide variety of free books that are out of copyright. It also has some deeply discounted books. Some books offer a free sample. When you buy a book you can specify which device you want it sent to and it will appear there. Other devices can download the book on request.
This is not quite as mature but it is close. It offers more viewing options than the Kindle app. In addition to changing font size, you can also change the font itself. Nook also offers more color options: white, sepia and eggshell plus white on black.The sepia option in Kindle is subtle. With Nook, it is more pronounced and closer to the color of old paper. Using an older-style font with sepia is very close to the experience of reading a real book. This is enhanced by the page-turn animation which is more pronounced than the Kindle. By touching the upper right-hand corner you can add a bookmark which looks like a folded over corner. It also shows the current page number and the total page count at all times.
There are a few things I don't like. The opening screen does not auto-orient. It has a fixed portrait orientation. To turn a page forward you tap the right side. This means that I cannot turn the page with my thumb. I have to hold it with my left hand and turn the page with my right hand. The animation is also a bit slow. Making the app look and feel like a book is a good thing but it should not affect usability.
Unlike the other readers, the Nook app opens on a menu instead of your current bookshelf. It does sync books between devices.
The Barnes and Noble store is not as tightly integrated. Buying a book does not push it onto a selected device. You always have to pull it in a separate step. Also, you have to enter your billing information and credit card before you can download a free book. There are differences in the books carried. I considered getting a digital copy of the Lord of the Rings so that I can reread it without carrying a big hardback. Amazon offers the individual volumes separately at $10 each ($30 for all). Barnes and Noble has a single-volume collection for $19. B&N also has a selection of recent books that are either free or $0.99.
Borders was late to the EBook market and it shows. They do not have a proprietary device. Instead they use a reader from a third party called Kobo. I think that this is where their desktop and app versions come from, also. Page orientation works but there are fewer display options. You can change font size and choose serif, sans serif, or monospace. There is no sepia option, just black on white or white on black. You can flip pages with your thumb but there is no animation. The whole page just changes. I have used this the least, partly because the lack or page flip animation make it difficult to tell that you changed pages. The app does sync between devices.
The Borders store is very similar to B&N's store in set-up and content.
Unlike the others, this is an open source reader that is not associated with a book seller. You can buy books through it. You can also use it to search Project Gutenburg. You have to manually rotate the screen with this reader and it does not sync between devices.
This is an app for reading files in RTF format. I have a copy of The Hobbit in this format so I tried it. This app uses a background that looks like paper complete with texture. It does not sync between devices and does not support any library.
Bottom line - I listed these in order of preference. Assuming that I can get the same book for the same price, I would choose the Kindle app first and the Nook second. I am avoiding the Borders app because of the page turn.