Hollow Knight is pretty great overall.
It utilises a really well done hand drawn art style, with character, enemy and environment designs that are unique and grimly cute. Its ambience and music are also pretty good, lots of tunes to get you hyped for an epic battle, to unsettle you as you trudge through hostile environments, and to just let you take in more serene moments. Overall the game has a somewhat dark and esoteric feel to it. The story of the world is quite intriguing, and is told through pieces of information gleamed from the diverse and interesting characters and by prodding around various points in the world. The full map of the game is large, and the individual areas are diverse in design and theme, and are also connected to each other in interesting ways, if you can find the shortcuts. Exploring the world is a lot of fun overall, usually done by making very good use your movement abilities to traverse platforming segments that range from easily doable to downright sadistic. Being thorough in your exploration can reward you with new or improved abilities, interesting information about the world, useful items and alternate pathways, which is always nice. The combat system is pretty simple, you start with a basic slashing attack with your sword, and can slash in any of four directions, and eventually get some extra abilities added to your arsenal, but battles are typically fast paced and can require good timing, reactions and strategy. The game world is populated with varied enemies that range from barely threatening, to highly lethal, with the most powerful regular enemies having damage, speed and defensive abilities that could even rival some of the bosses. This game has boss fights by the way, quite a few, and most of them are epic fights against powerful foes that will require you to showcase the best of your combat abilities, with the game's great soundtrack assisting in increasing the awe factor of these battles. Add in responsive controls and a smooth movement system, and you've got an overall intense and highly enjoyable combat experience.
This game isn't without its flaws sadly, but I found them easy to get over. The world isn't exactly dense with fast travel points, so if you want to revisit specific parts of areas, you're going to need to be ready for a few treks. Can be mitigated somewhat by finding shortcuts, but not completely. The game's checkpoints, which you respawn at after death, can also feel like they're in rather short supply sometimes, particularly in more difficult areas.
In any case, I'd recommend this game greatly to anyone, particularly those fond of Metroidvanias and 2D action games.