My only warning is: you MIGHT end up wanting to use your device for more than just watching TV shows/movies--and if/when that happens, you'll find yourself severely limited by the Kindle. If you're hooked into the amazon ecosystem with Prime, it makes more sense to get one (though, I have Prime and an N7). I don't know about the Fire HD, but I couldn't get external keyboards or external memory to work w/o rooting on my parents' Fire 1.0's.
The Kindle launcher is butt-ugly; no widgets or customization of any sort, just a carousel of app buttons. It's very intuitive for people like my parents, as it does give each book/movie its own button, but it's a pain to scroll through.
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This is a screenshot of my N7:
^Not the prettiest, but it's exactly everything I need to access, all conveniently there. I have 3 homescreens; one with newsfeed widgets; my main homescreen as above; and one with my email/tasks/calendar widgets. I take it everywhere with me, so no only is it my media consumption/note-taking device, I use it to plan/organize my day, I use it as my GPS, etc.
This is a screenshot of Amazon's carousel:
Just watch some YT video comparisons to get a sense of what each can do. I'm already biased because I have an N7
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