This game is both a work of art and passion. The amount of effort and detail placed into each enemy is truly staggering. The animations are fluid, hand drawn, telegraphing and is complimented aptly by a great musical score. It's a tightly constructed package with no shortage or excess of ideas. Each boss encounter feels unique and memorable. The best design of the game is how it creates a sense of progression through player skill.
Game play is simple but challenging. You die in three hits and most levels and boss encounters quickly devolve into bullet hells. You can jump, shoot and dash. While jumping, if you hit the jump button again while in the air, you can parry any pink object. I think it was a pretty apt decision to color code all parry-able attacks as pink. With how chaotic a boss fight can get, having visual clarity goes a long way for an enjoyable player experience. Parrying felt really awkward, particularly if an attack comes from the bottom of the screen. You have to have extensive forethought to jump ahead of time, and before you land on said attack, you then hit the parry button. It's really easy to push the parry button too soon and get obliterated. Not an elegant system, but I really liked the inclusion of the mausoleum events as parry tutorials. Without them, I may never have gained a grip on the mechanic.
You gain various upgrades and weapons via the store, but they are more minor tweaks. For example, the peashooter does 4 damage per hit and has a high rate of fire whereas the charge shot does 6 damage uncharged and 46 damage if you hold fire long enough and thus has a significantly lower rate of fire (I felt like this was the best shot overall. Made it pretty easy to cleave through bosses once you get a feel for it). You can beat each boss in the game with your first weapon, except for maybe the Phantom Express (his first phase probably requires the scatter shot, at least that's how I beat it) and the last phase of the Bee boss seems way too hard without the lobber shot. The trinkets you can equip are mostly useless except for the smoke bomb, which gives you I-frames whenever you dodge. You basically always want to use this which is kind of a shame that the devs couldn't design the other trinkets to compete. You have three super abilities. I found myself using the blast attack the most. The invulnerability I used for King Dice, and that's it.
My only issue with the game is the sheer amount of RNG that gets thrown at you during a boss fight. Not all damage feels avoidable. Sometimes it feels like you just get screwed over with multiple attacks/enemies stacking against you at the same time. Still, it mostly works and most damage feels fair.
Overall, I'm glad I finally sat down to conquer this game. It took me a little less than 8 hours. The hardest bosses for me were The Phantom Express and King Dice. The later was challenging due to the attrition. I think each mini-encounter was well designed in a vacuum (except for that damn chimp with cymbals! Fuck that guy!), but it can be terribly draining starting this fight over because of how long it can last. Easily the longest encounter by a huge margin in the whole game. I did feel like the Devil was a bit disappointment. He wasn't a push over, but compared to the penultimate boss it felt too easy.
Addendum: I just beat the DLC. I don't have much to say about it. The DLC provides more of the same type of content without any innovation. I did find the new "charm" or playable character to be strong. She has 4 hearts by default, an invincible dodge roll ( which can be a little awkward to use since you need to be on the ground) and a dedicated parry button. I really like the dedicated parry. It takes some getting used to since it is a dash into parry, but it makes parrying significantly more reliable which is a huge improvement from the base game. She does feel overpowered though. In addition, the crackshot and lightning shot are insanely strong. They easily outshine the charge shot. I don't think the DLC is a must play, but I don't regret it. It only costs $8, but it does still feel very short. I was able to beat all the bosses on normal in about 2-3 hours. Still I don't feel bad supporting a creative team.