I have had Prime for many years... but have never used the video part of it.
Main reason? The lack of any place to play it. None of my TVs or my new Apple TVs support it. Sure, I could pull out my iPad... but why bother... plenty of other places to watch stuff that are supported right on my ATV.
Plus their video quality used to be pure crap when I did try it once... I've heard it's improved a lot lately though.
If they ever release a tvOS app then I may take a peak...
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The thing is... I don't WANT another decice to deal with at all. Getting my wife to press the ATV button on our remote is hard enough... no way is she going to want to mess with yet another device. There is just too much friction involved with dealing with multiple devices.
They have very good Picture Quality now, one of the few to actually have 4K HDR. I very much agree about needing another device. I have an ATV and then use google cast for everything else. Amazon is not available on either of those options. I will not buy a fire stick, roku, etc for just 1 service.