I purchased this from the US before the UK version came out and I'm glad I opted for the larger 8.9 inch version than the smaller 7 inch Fire HD as there is a marked increase in resolution and screen real estate (this is better than full 1080 HD). I also have an iPad 4 and this review is based on a comparison of the two tablets as I believe they are the top two (Samsung Tab would be number 3) in the market.
Size and build quality: I like that this is slightly smaller than the iPad and most importantly it weighs quite a bit less, helping you to hold it with one hand (not for too long though). Obviously the screen is the most important aspect and being made of glass you will always worry about it breaking on a fall (get a case!) but it looks well put together and nothing feels as if it is made from cheap plastic/other materials. Has a solid feel when you pick it up but definitely not as heavy and bulky as the iPad (Kindle feels thinner). Kindle wins on this one.
Display: Almost perfect colour reproduction, text clarity and video. You will not notice anything deficient in the display, it performs excellently with everything thrown at it. I like to read books at night in white font on a dark background and then switch to the browser with dark text and white background and have never had any issues when making the switch instantly. Likewise watching movies, even something like 'The Dark Knight' which has very dark scenes was perfectly well reproduced with no discernible difference from a TV (provided you use the HD version of a movie). As there is a built-in mini HDMI port you can connect the Kindle to a TV and share what is on the screen or play a video which is very handy. The ONLY slight negative is that the iPad retina display just has that added wow factor that means I always favour its display to others, because of the higher resolution and technology which reduces the layers to make the screen sharper. In all though there is not much to choose between the two for 99.999% of people and uses.
Audio: Definitely a winner here, the stereo speakers pack more punch than you think and is definitely the better performer when compared to the iPad. Obviously not as good as a state of the art hi-fi system but for a mobile device this is as good as it gets. Mid-range is good and audio comes across with great clarity, you won't get distortion unless playing something crazy from YouTube. Kindle wins here.
Wifi/Browser: I got the wifi version (16gb) and can honestly say it holds a signal better than the iPad 1-3 but no difference with the iPad 4. The web browser is not as snappy and the system runs slightly slower than the iPad but you will only notice that if you are running them side by side, otherwise it's pretty fast on all fronts. Obviously no flash, but the web browser does the business, maybe lags slightly behind Safari and Chrome but not noticeably.
Books: Because the screen is of high quality reading is a pleasure and the integration with the Amazon store is brilliant so you can find books and download very quickly. Text can be enlarged and fonts changed to suit your reading style, no reason why ebooks cannot be compared to normal books now in terms of how you read the text. Page turning animation makes it feel like you are reading a paper book and the fact that you can switch to another title so quickly means you are now truly in control of a library. Performs as well as the iPad in this regard.
Apps/games: Don't respond as quick as the iPad nor are there as many choices, but should have everything you would use regularly. I can't say the store experience is better than Apple either but it did feel that this will only improve with time. iOS is ahead here.
Bluetooth: Connected to headphones just fine and played music in stereo, can also be used to connect a keyboard which is a better option for productivity (see below). Worked with a bluetooth headset as well when making Skype calls.
Email/typing: Touchscreens are not well regarded for either use and this is the case here, not good for more than a few lines and you will get regular errors. The iPad is the better performer here as the keyboard is organised better (and better spaced) and less errors are made. Kindle behind here.
Battery life: The 10 hour claim is about right, obviously reading a book in plain text works the tablet less than say a video and you will get more use out of it that way. It doesn't get as hot as the iPad so definitely an advantage here with roughly the same running time. Only a USB charger is supplied which is a definite negative as it takes well over 12 hours to charge fully from a USB port. The iPad has a wall charger so Kindle is behind here considering you need to pay £13 to get one.
Camera: front facing camera is not as good as on the iPad but does the job for Skype calls and any other apps that require video. Definitely not for taking photos. There is no rear camera, so the iPad wins on this front.
Memory: 16gb was fine for me and the Amazon cloud storage should cover you for most of the future purchases (it only covers Amazon purchases and content, other content will eat up your free allowance). Books don't use up much storage space, so you should get in well over 2,000 books on this without a problem.
Price: Around £170 cheaper than the iPad and definitely more value for what you get, hands down winner on that. Buying from the US was quite a lot cheaper than what it is being sold for in the UK. At £229 I think that Kindle Fire HD 8.9 offers the best features/price ratio against major rivals and makes it easier to justify the purchase for this sort of device.
In summary, this is a very good tablet, number 1 for me if price is factored in. Minor gripes about the charger not being included, ads being intrusive sometimes (you can pay £10 extra to get rid of them) and app store still weak compared to iOS do not really affect the device overall. If you are picky you will knock off one star (especially if you compare to iPad 4) but most people will be very happy to pick up a high quality tab for £229. In conclusion, best tablet for the price and if this is your first one then you will not be disappointed.