Without a doubt, Horizon Zero Dawn is still one of the best games I've ever played. Fantastic visuals, great gameplay, incredible story… this is the complete package if anything ever was.
I had some trouble getting a working copy of the Frozen Wilds expansion and eventually did so by getting this "complete edition" which includes the base game and the Frozen Wilds expansion in one. Once I finally was able to play, I started from a new game and enjoyed it just as much times as the first two playthroughs. There were a few new skills, a few new characters, and of course the entire new area with new machines, new contacts, new stories, new collectibles and new challenges.
Story-wise, it perfectly meshes with the existing narrative in a way that makes it seem like a part of the original game – exactly what you'd want for an expansion. It gives some new weapons and armor that seem overpowered by comparison to the base equipment (and even the best options in the main game), but that's ok because the level requirement for Frozen Wilds makes it a good idea to only tackle when you're most of the way through the main story or at least well equipped and experienced. Despite that I still had some trouble with the machines there so fair warning: they're beastly.
The only downside I could point to was the ridiculous arm and neck waggling the characters did in talking scenes. It was like watching someone who mistakenly thought they were supposed to animate power rangers (who can only express emotion through constant flagellation of their bodies). It was out of place compared to the rest of the game, but really is a piss-poor animation style and is out of place anywhere.
I suppose most people wouldn't care as much as I did and I admit it was a very minor annoyance in an otherwise flawless game so I still recommend it. If you like great story-based single-player epics, this is as good as they come.