Alright, my (wife's) Kindle Fire HDX 7" arrived yesterday and I got to play around with it a little bit last night.
As someone mentioned above, if you dislike the native UI (which after a few hours I actually found to be an acquired taste), you can very easily install a third party market (1mobile in my case) and then install your favorite launcher. I installed Nova launcher from 1mobile and just like that there's the standard android home screen I'm familiar with. Home screen, dock, app drawer, everything is there. A little bonus is that right in the middle bottom of the screen there is this tiny tab that when you pull it up shows the original carousel that's the main feature of the native UI. So yeah, you don't even lose that handy shortcut (it's basically a pretty 'recent apps' list) using a different launcher.
The only issue I'm experiencing with using a third party launcher is that I couldn't find a way to set the wallpaper, and no matter what I set it to from Nova setting it stays the default black screen. I haven't really looked into this though, so maybe there's a way to go around this restriction.
Everything is fast and super smooth with this tablet, which is to be expected considering its specs.
One important discovery for me: I was a non-believer of the 7-8" tablets. I thought they're useless. I've had a 5" phone since 2010 so to me a tablet should be at least 9-10" or I won't bother as everything else is not that much bigger than my phone. Now that I actually tested one, I found that it's the perfect size for personal movie viewing.
This tablet seems to be exactly what my wife wanted. If I were to get a tablet for myself though, I'd still rather get either the Galaxy Note because of the digitizer, or a Surface Pro/something similar.