The differences between the 7-inch and 8.9-inch models are about as subtle as the differences between the iPad Air and iPad Mini. The Kindle Fire HDX 8.9 has the same software, apps, and services as its little brother, but packed into a bigger body.
So I'm not going to spend a lot of time going over the same stuff I did in my last Kindle Fire HDX review. Instead, it's more important to focus on what sets the Kindle Fire HDX 8.9 apart from the smaller model and the rest of the competition.
Big Screen, Light Body
I've grown to become more accustomed to the tiny tablets, but in this case, I like the Kindle Fire HDX 8.9 a little better than the 7-inch model. It has all the same design schemes, but it's incredibly light for a full-sized tablet. At 0.82 pounds, it's only slightly heavier than Apple's iPad Mini with Retina display, and noticeably lighter than the new one-pound iPad Air.
That's my favorite part about the tablet. It gives you the full-sized experience, yet it's light enough to use anywhere — in bed, on the couch, on the train — without feeling the weighty burden of other big tablets. Yes, part of the reason why the HDX 8.9 is lighter than the iPad Air is because it's slightly smaller, but I didn't miss the extra 0.8 inches of screen. That's a sacrifice I'm willing to make.
And the screen is gorgeous. It's technically sharper than the one on the iPad, but you probably won't notice a difference. One study said the Kindle Fire HDX has the best screen of any tablet. (I think the screen is great, but I'm not about to go through and count every pixel like others have. Just know you'll be happy with it.) Just like other high-res displays these days, images, text, and graphics look like they're printed on the glass. Movies, games, and everything else look great.
The HDX 8.9 is made out of plastic, but it's a high-quality, rubbery plastic that doesn't feel cheap or flimsy like some other mobile devices. It's not beautiful like the iPad Air, but it is perfectly functional. The only flaw in the design is that the volume and power buttons are on the back of the device, which makes them awkward to use. You'll have to develop some serious muscle memory to get used to it.
The Kindle Fire HDX 8.9 is technically an Android tablet, but Amazon has modified Google's OS beyond recognition. Amazon stripped out all things Google from Android, meaning you don't get access to great services like Gmail, Google Maps, and the Google Play store for apps. (Yes, you can plug your Gmail credentials into the Fire's email app, but the experience isn't as good as Google's own Gmail app.)
That's the biggest bummer with the HDX 8.9. The Google Play store has a much better selection of apps than Amazon's own Appstore. Developers have to submit their Android apps separately to Amazon if they want to be on the Fire, which results in a bunch of apps that are either out of date or late to the game.
But when it comes to books, music, TV shows, and movies, Amazon has you covered. You'll find Amazon's content selection is on par with anything you'd get from Apple on the iPad.
In short, the Kindle Fire HDX is at it's best if you want to use it to consume content directly from Amazon. But if you need the latest tablet apps and games, you're better off with the iPad or an Android tablet like the Nexus 7.
There's one other bonus on the Kindle Fire HDX: Mayday. Mayday is a service that connects you to an Amazon customer service representative over video chat. (He or she can't see you, but you can see him/her.) Mayday reps can help you with any problems you have with your tablet, and can even take over your tablet to show you how stuff works. It's an interesting alternative to Apple's "Genius" customer service model, which requires you to go into an Apple Store to get help.
Like I said before, if you're trying to choose between the 7-inch and 8.9-inch models of the Kindle Fire HDX, I say go with the 8.9. It's $150 more expensive, but the bigger screen and super-light body is totally worth it.
But I'd recommend against buying any version of the Kindle Fire if you plan on doing anything beyond reading books, watching TV shows, and doing some light emailing. Like it's predecessors, the Kindle Fire HDX 8.9 can't stack up to the iPad and other tablets when it comes to app selection and productivity.