Added a KF to a household with multiple Windows laptops/desktops, a couple of Android smartphones, and an iPad 2. So comparisons, especially to the iPad 2 are inevitable. Cutting to the chase, the KF provides about 80% of the functionality of the iPad at 40% of the price. As far as specifics are concerned...
Aesthetics: Very good. A bit heavy for its size but still 30% less than the iPad and much more portable. Nice look and feel. Added a Marware "CEO" case and it eliminated a nagging concern about dropping it.
Screen. Excellent for media consumption. My wife thought she'd miss the e-ink display of her 3rd gen Kindle but tweaking the brightness works great. When and if the sun ever appears in Seattle she may prefer the Kindle to the KF, but it's a minor issue. Otherwise, the 16x9 aspect ratio is great for watching movies. Better than the iPad's 4x3 format.
Hardware. Purchased after a couple of Amazon updates to the OS and the hardware works very well. One experience with an unexpected shutdown in a month of ownership but a hard reset resolved the problem without difficulty.
I'd prefer a hardware based volume control but it's not a big deal. For me, at least, having the audio output on the bottom is a bigger PITA. And having the on/off toggle on the bottom hasn't bothered me at all, though I can understand complaints along that line.
Overall User Interface. I don't like the fact that I have to remove items from the carousel manually. It's a PITA. And the Favorites shelves should be at the top of the screen, not the bottom. Haven't tried sideloading a different launcher. May try that. Otherwise, if you're accustomed to an Android UI approach, it's a piece of cake.
Widgets. N/A
Signal quality (Compared to other devices). Excellent wifi connectivity on my home network. Connecting to my Verizon mobile hotspot is sometimes a challenge especially if other devices are already connected. Requires turning hotspot on and off multiple times to connect at times. Otherwise, it works well with public wifi's (though I don't use them much.)
GPS. N/A
Social Networking. Mainly N/A for me. Facebook is OK but I seldom use it.
Email/Calendar. Excellent compared to our cell phones. Not nearly as functional as a full-fledged email client. Sync with google and outlook is more or less a nobrainer.
Battery Life. Not outstanding, especially compared to the iPad or other Kindles. But neither is a fair comparison. For us, it requires recharging only at night so that's fine.
Performance. Excellent. Haven't experienced any of some early reviewers' complaints about lags, jumpiness, scrolling issues. I suspect software updates for the OS and the browser have something to do with that. Browsing using the Silk browser is fine. Not earth shattering in either accelerated or non-accelerated mode, but no complaints.
Media (music, movies, DLNA). Speaker OK, nothing more. Drives a pair of headphones very well, though. Movies are great! As noted above, the 16x9 format makes up for the 7 inch screen compared to the iPad.
Keyboards. I hate virtual keyboards. The KF's is about average. Not as "good" as the iPad but that's mainly because of size. I'd much prefer bluetooth support for a real keyboard but I doubt I'd use it much given the other devices in the household. Haven't tried sideloading other keyboards but having Swype or another of the popular keyboards (e.g. SwiftKey) available on Android would be nice.(I assume that can be done with sideloading or rooting.)
Camera (both still & video). N/A
How the Kindle Fire compares to your previous tablet (if applicable). Considering the price point, comparing the KF with other tablets is problematic. Would I rather have the KF than an iPad (or one of the most recently introduced Android tablets?) No. Would I rather have a Miata than a Ferrari? No. For my requirements (including content creation) the iPad is a better choice even at a price premium of at least $300. But my requirements are pretty stiff. As a content consumption device at 40% of the price of an iPad, it's a bargain.
Disappointments (if any). Sure. No SD card support for more local storage. No bluetooth. No option for an e-ink dsplay. A bit of a hassle to enable sideloading of additional apps. Hate the design of the Carousel. But none of these are deal breakers for me or the rest of my family. Would I pay another $100-$200 for these and other enhancements? Nope. All in all it's a GREAT device.