Yes I have used it, but I find I prefer to read books in comfy chair, rather than sitting upright at a computer. I have the app for android, and use that a lot. Android app and app for PC are not as easy to use or intuitive as using a kindle, but they do the job. And they are free. My kids have "borrowed" my kindles. I don't mind as it encourages then to read, so I use the Android App, on my cheapo android tablet. And there are some real cheapies out there.
If I lived in the USA, I would consider buying a kindle fire. There is so much content for the Kindle fire, I think I would buy it for that. In the UK, the copyright laws prevent much of this content being downloadable. Stuff can be streamed but not downloaded. So a kindle fire needs to be tied to a wifi point to get the best out of it. Other than books, there is little downloadable content. What is the point of having a highly portable device, that needs a wifi point to use?
There is masses of downloadable stuff for Android. According to the reviews on the web, various reviewers have commented, that Amazon has knobbled the Kindle so it will not perform some Android functions, so I would be unable to download the Android stuff to a Kindle Fire, but I can to an Android tablet.
On top of that the Kindle fire is 2-3 times the price of a cheap tablet, and the same price as a quality Android tablet.
Can any one wonder why I wont be buying any more kindles, perhaps a better Android tablet.
Just my two pence worth
Anrol
PS In the US, for 50 dollars, you get books, plus films, games, apps, etc., so a Kindle fire may well be worth it.
In the UK you pay 50 pounds a year for Amazon prime, to be able to download a library of books for free. I had a look at the titles, they are the ones you pay 99p for, not titles from major authors.