Allow me to dump my own personal review that no one asked for.
Playing the game, I got a lot of Sunshine vibes. The controls are easy enough to get into, and while some things (namely the occasional camera fuckery or the odd homing attacks that require you to use your refreshed jump every time you hit a target) can be iffy I never found myself getting *frustrated* when dealing with them. Hat Kid felt satisfying to control, in my opinion. The levels are large, and while for some that might be a complaint, I found it made for satisfying use of movement mechanics (IE, if you press the dive button, then press it again a moment after Hat Kid touches the ground, you'll get a small speed boost). Each act in a chapter tends to change the levels in various, albeit, inconsistent, ways. Chapter 1 is more or less the same, just with different environmental conditions, and then it's flooded with lava in one act. Chapter 2 was amazing with the train levels. Chapter 3 kind of slows things down again for a more somber atmosphere, and Chapter 4 does the opposite, inviting you to gather 4 time pieces in one go (It's almost a hub world in its own right. The level is massive with a central area, 4 large sub areas that cover different elements, and then a few small sub areas for other small goodies) before dropping artillery on you to try to stop you from getting a 5th. Personally, I loved this. There are some other small nuances in the level design, too. The game makes excellent use of its verticality in regards to straying from the beaten path. I usually never bother with anything that's not my "main quest," but oftentimes here I found myself curious just to get whatever goodie was hiding around the corner, and while with a lot of games I'd say something like "fuck, I have to go back after going through all the trouble of getting this," most often I found myself either able to literally dive back to where I was, or finding some other shortcut to get back.
I don't particularly have any gripes with the lack of multiplayer or for a system that is effectively there for aesthetic reasons– as shallow as it is, being able to get colors and hats helps in making your game feel like it's yours. Multiplayer is supposed to be patched in along with Chapter 6 with the next large content update, if steam forums are anything to go by.
For only having 40 time pieces though, the game sure felt like a rather wholesome experience, at least for me. I wound up clocking 10 hours on it, going for 100% at a rather leisurely pace. I actually picked the game up while I was in a rather sour mood and it cheered me up quite a bit, both in regards to its gameplay and its dialogue. For 30 dollars, I'd say it was worth the time I spent on it, at the very least. It accomplished what it set to do at its core: be an homage to GC era platformers, while also standing on its own. For Mecha the Slag's first earnest foray into game dev, I'd say he shows much more promise than most other devs in the >current year.