I have an HDX 8.9. I actually love it for the combination of form factor and performance. It's lacking an SD slot and mine has been heavily modified from its stock software load, but more importantly to me, NO ONE has replacement parts for it, and it's old enough that the battery has deteriorated from a new model. I loved it to death, apparently.
The thing is held together with glue and as I understand it, HDX 8.9s are nightmarish to work on, so I'm just going to have to live with a device that can only manage 4 hours of screen-on time and hope that Amazon eventually gets around to offering an updated model of similar specs.
And that's the problem, because the product line hasn't been refreshed in two years or more. New Fire HDs don't live up to the hardware specs of the HDX. They're the wrong sizes and weights, have lower screen resolutions and slower CPUs. Even as somebody who isn't bothered by having to side-load a significant number of apps to overcome the shortcomings of Amazon's app store, it's a drag.
I have an nVidia Shield (another tablet that is no longer being manufactured) that I also like well enough, but it doesn't quite live up to the screen of the HDX. Samsung's high end tablets typically have gorgeous screens but never, ever get OS updates. Asus and LG make 8"-class devices are likewise not quite satisfactory.
I'm sure Amazon sells a lot more $200 HDs than $400 HDXs but damn, some of us actually do want one.