Sounds like you've been conditioned to the Fire OS - especially if you're a repeat customer coming from a 1st gen. You're already used to it, and getting a recent device is a nice upgrade - no doubt about that - but as for the rest of us...
It's not unusable by any means but it's a mess of an OS usability wise, the app store is bad, the web browser is bad, the in-hand feel is weird (talking about a 7" HDX with the beveled back), the power/volume buttons are horrible, and I could go on if I still had it around but it's been returned as of about a month ago. I compared it directly against a 2013 Nexus 7, and once I got that HDX delivered I found all new appreciation for the Nexus at the same price point. It ended up convincing me that the 2013 Nexus 7 is the best cheap tablet out there, but the one notable win for the HDX was its screen. It really was a lot nicer, but the 7's screen is plenty good on its own.
Call it whatever you want, but the experience on the 7" Fire HDX was just full of cons compared to a Nexus 7 and fared even worse compared to an iPad. It only took me about a week of back and forth to know that choosing the 7" HDX over the Nexus 7 would be a bad decision. So, from my standpoint, on a budget a Fire doesn't make much sense when a 2013 Nexus 7 can be had for under $200. Along with that, from a premium perspective, an HDX 8.9" Fire doesn't make a lot of sense because you could spend about the same or a little more for an iPad, and then have access to pretty much every Amazon service along with the superior iOS app store - which includes Google services that Amazon doesn't offer and everything else under the sun.