Just an update. After about 3 or so hours I can confirm that Hollow Knight is a legit old school treat. It's not an "easy" game, but it is fair. You don't have the ability to just hack and slash through bosses like a lot of modern games. Most bosses take timing, patience, and pattern recognition. (as they should!).
Bosses and enemies that feel (impossible) at first become completely easy once you learn there pattern. Plat forming (so far) hasn't been that tough, but I can see the mechanics are in place for some tricky plat forming sections.
The game can feel a little overwhelming while you're exploring, because save spots aren't clearly marked at first, and maps have to be found and updated.
The key to the game (so far) is pacing yourself and healing yourself on a regular basis. As you progress you start to earn "badges", that act as buffs and help to make the game a little easier. The issue is that you can only equip so many badges at once, and you can only change badges at save points.
This makes for interesting choices. For instance one of the badges is a "compass" that basically acts as your beacon on the game map. Without it equipped, your left just staring at a map without knowing "exactly" where you are. So it becomes a choice. Do I buff this ability, or should I equip the compass so I don't get "lost". The lack of save points, or save anywhere feature adds to this dilemma. When you die, you not only go back to the save point, but you also lose all your "money". The good news is you can return to the same room you died and "kill" your ghost, which gives you back all your loot. (Similar to dark souls?, I 've heard.).
Anyway. Highly recommended for fans of old school Metroid/Castlevania/Mega Man games.