Reached 100% on both the main game and the Frozen Wilds DLC (PC) and it was actually one of the best single player gaming experiences I've ever had.
Story: The game can be really hard at times but the story makes up for it. The story is engaging from start to finish and always makes you wonder what comes next after each quest. It's that good.
Gameplay: Gameplay is not amazing but it's pretty decent. Dodging can be a bit frustrating at times, considering some machines actually do have some serious OP attack combos but you'll get used to it eventually and will end up having fun. It feels kinda nice seeing canisters and components flying of the machines you're battling against.
Graphics: Yeah this is where the game really shines. I played it at 1440p with max settings and it was quite the experience. The game has some impressive vistas across the board. Mainly the desert areas. The DLC also features some incredible weather and snow deformation effects that were eventually included in the sequel (Horizon Forbidden West). The game started as being not that good of a port but as of right now, it runs pretty well overall.
Difficulty: Yeeaah, the game is HARD. You can choose the difficulty that best suits your play style but the game relies heavily on planning and careful strategic execution. If you go guns blazing into a herd of machines you'll get most likely slaughtered. But even if you do approach combat carefully, it can still be unpredictable. Just take your time to study every type of machine and make sure you have the most suitable weapon loadout for each occasion.
Conclusion: The game is incredible. Not perfect, but still incredible nonetheless. Fully recommend!