If you don't have a Kindle but you have an Amazon account, you can still download and use the app to read Kindle books, and I recommend that you do so if you don't mind reading on your PC or laptop screen. The biggest benefit is you can take advantage of the tons of free books— not only public domain works but books whose author are giving them away as a promotion— that are available oin the Kindle store. Second, if you happen to be visually impaired, the app makes every Kindle ebook a large print book, and that can be a real benefit.
First the bad news. This is a beta version, and it shows in the fact that a few things you can do on the Kindle are missing on the PC app. You can't annotate a book (although if you have a Kindle and made notes in it already, you can read them). You can't search the books. Amazon plans to add both those features plus zooming on images in future releases. There was no mention of showing location numbers or percentage of the book that has been read, two things you see on a Kindle screen but not on the PC version. It must know the numbers because it has a Go to Location function.
The good news? Well, besides the obvious (it's free!), buying Kindle books with it is dead easy because once you register, you get a "Download" button in the Kindle store; if you have a Kindle already, the PC becomes another choice on the drop down. Whenever you buy Kindle books, they go into your personal archive. That archive is available to the PC app, and books download from it even faster than on a Kindle. Both your archive and your home screen (the list of books downloaded) show color thumbnails of the book covers, along with the title and author, and when you open a book, the cover is in color. My 97 archived books showed up and looked great on the screen. None of the magazine issues showed up, though. I don't know if Amazon plans to add them or not.
Also, the PC app offers more font sizes than the Kindle, and the largest one is honking big, two or three words on a line. I can read without my glasses, which is amazing.
The interface is clear and intuitive. You double click on a thumbnail to open a book. When I opened a book I had started, it took me right to where I had left off reading. Some of the things are very-Kindle like, such as the Menu options on the Home screen. Others are not; there is no Menu button while you're reading a book, and Shop in Kindle store just takes you there in your browser window.
Creating a bookmark dog-ears the page display, just as it does on a Kindle. You can sort your downloaded books by author, title, and latest read, just like on a Kindle.
I think Amazon was smart to release this version so early, and mark it as beta. That way they get feedback while they're still doing development. Plus, the eReader market is so hot now, they need all the buzz they can get. And the free PC app (it works on Windows XP, Vista and Windows 7) may well lure in folks who are interested in eReaders but not willing to invest in one without kicking the tires. After all, except for Mac users, who doesn't own a PC? And the Mac version is coming, too, so don't feel left out.
If I had to pay for it, I would give it a B- but since it's free, I'll give it a B+.