Absolutely wonderful, one of the best designed games that I've played in a while, a lot of indie games shoot for Super Metroid / Dark Souls I don't think I've played one yet that nails the gameplay design of those quite as well as this.
What's great
Nothing exists in a vacuum, EVERYTHING the player does has consequence, and it nails that mini metrovania / dark souls "skillful" level up feeling perfectly. The game doesn't get easier, you just understand it better. The game does a great job staying consistent with it's internal ruleset and rewarding you for understanding it better.
You memorize optimal bullet / whip / throw combos for low level enemies, you make decisions about whether you're going to quickly finish off an enemy with a quick shot or take a slightly more risky approach to harvest some ammo, later you might learn that you can do cool shit like take the time to harvest one enemy for ammo and use them to kill another enemy to more efficiently get more ammo.
Small niggle
I might just kind of suck but I wish that there was more unique markers of where you were. Obviously a full blown map would sacrifice a lot of the good feeling of exploration that the game has and discourage keeping a mental map of surroundings, but I think that if you're going for Dark Souls it would be nice if areas were named and there was a little area on the HUD that displayed the name of the current area / room that you're in.
This I think would be
1) pretty easy to implement (other than coming up with thematic / creative names for rooms)
2) Help players draw physical maps or keep better mental maps
3) Let frustrated players ask their friends if they get lost / stuck or check online and get good instructions / advice instead of just quitting for good or having to describe where they are stuck in very subjective terms