Appstore for Android
Android Market is not available on the Kindle Fire, replaced by Amazon's own Appstore for Android. Lest you forget, this is an Amazon device, not a Google device. Nowhere do you see Google's name explicitly, other than the default search engine in the browser.
There's 10,000 apps instead of something close to 400,000 in Android Market, but Amazon's store has a pretty healthy catalogue to choose from - every major app is available, though some of the smaller and more random apps are not available. Netflix, Hulu, OfficeSuite Pro, Documents To Go, ESPN Scorecenter, Angry Birds, Words With Friends, Evernote, eBay, IMDB, Chase Bank, Fruit Ninja, Atari Games, even WiFi Analyzer are available. I haven't found anything important lacking yet, but if you're addicted to getting every single random app, you're probably going to find the Appstore selection a bit on the smaller side. I do like that every day, there is a featured paid app of the day that is offered for free. I, like most people, enjoy free things, so I'm not going to complain.
The apps themselves are standard Gingerbread ones - the experience feels pretty similar to the 7" Galaxy Tab. They're blown up smartphone apps, really, just like they were on the Nook Color and Galaxy Tab. There are questions as to what will happen if the Kindle Fire gets updated to Honeycomb or Ice Cream Sandwich, but that's something that will have to be monitored in the future.
Documents & Email
Documents is pretty similar to Books, though it's with content that doesn't come from the Kindle storefront. Personal documents can be sent to the Kindle Fire via something called a Send-to-Kindle email address. Only approved contacts can send documents to the Kindle Fire. Documents can be in a number of formats - .doc/docx, PDF, HTML, TXT, RTF, JPEG, GIF, PNG, BMP, PRC, and MOBI files. What this means is that if you play it right, you will never need to connect the Kindle Fire directly to USB.
The email application isn't the same as Gmail, but if you set it up properly, it works similarly to the native Android Gmail app. It sets Gmail up as an IMAP account, so you do get push notifications. Obviously, no labels, archiving, or threaded conversations. After getting used to the features of native Gmail, this feels a bit like going back to the stone ages (or iOS). I wish there was an option to get the native Gmail app on the Kindle Fire, but I understand why they had to ship an email client with support for all email services.
For non-Gmail users, the email application is sufficient as well. I used it with our AnandTech IMAP servers without a problem. Deleting accounts is a little awkward (you have to back out to the accounts page, tap and hold and then remove account) but otherwise the experience is pretty straightforward. I've also found that the email app doesn't always respect you telling it not to automatically check for emails, which can be a bit disturbing if you're trying to do something else and new email notifications keep appearing.
The email experience is ok overall, it's no where near as good as the iPad - partially due to the smaller screen, but it is easier to read and reply to things on here vs a smartphone with an even smaller display. The Kindle Fire's form factor does allow you to have somewhat of a smartphone experience by allowing you to hold the device in both hands and thumb away at the keyboard.
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