I've been using an HDX 7" for the past few weeks, as part of a 30 day free trial.
I've been using android products/phones since the original G1, and am currently on the Nexus 4.
I've got to say, the Kindle Fire sucks. Its nice having a larger form factor vs a phone and all, and browsing is decent when the wifi works, but other than that, it's not worth getting it over a Nexus 7.
You are severely limiting the functionality of the tablet by getting the Kindle Fire because their app store is miniscule compared to stock Android app store. Sure, you can root and custom ROM the kindle to access it, but then why not just get the Nexus 7 and still maintain the warranty and skip the hassle at the same price point.
Wireless connectivity is spotty, even when "connected" internet/data access will be unavailable frequently (several times per hour) unless you cycle through wifi off/on or airplane mode on/off to reestablish data connection (other devices connected to same network does not experience this problem).
Also, the app store is pathetic, and what few apps are available don't work well, such as the Spotify app... It will only play one song and never continue to play the next songs, no matter what settings I use. There is no gmail app or map app, and the built in email client can't search your emails the same way that google can.
The battery life is quite good though, and I can go a couple days before having to charge it. The sound is really good too, with the stereo speakers in landscape mode.
However, for the same price, the Nexus 7 is surely the better deal. You get full access to the app store and all the updates from google directly.
Of course, this applies to the HDX, which when 244 without adds when I "purchased" it. The 100 bucks you paid for the original is a steal, and you could probably get 100x more use out of it by installing a custom ROM.