It's a bit of a weird one, though. Koumajou Densetsu looks and feels like a Metroidvania, but it's actually structured like a classic Castlevania game. In other words, it's back to being a pure platformer. There are even bottomless pits!
Frontier AJA has the Castlevania feel down to maybe the 90% level. The sprite of the main character, Reimu, is easily up to the level of Konami's games. She moves much like Richter Belmont from Symphony of the Night, and wields the whip much in the same manner. But it's clear the developers spent a lot of time focusing on her and not much else. The jumping feels a bit unresponsive, and the hit detection feels quite buggy. Since this is based on Touhou, cutesy fairies take the role of famous Castlevania enemies like spear and axe knights, though there are still several regular enemies, which have all been redrawn, but look a bit dull. The music, while fairly decent, is done by someone who obviously loves Castlevania music a whole lot, but lacks the talent to create something that truly stands on its own.
The worst of it is that Koumajou Densetsu borrows from the Metroidvania level design philosophy - that is, lots of flat surfaces, slopes, and not much in the way of variation, with enemies just kind of strewn about. This style is okay for Metroidvanias, because they're more about exploration than action-platforming. Obviously, when you take that style and apply it to a game that IS an action-platformer, it doesn't work. The designers will do nasty things like sticking four Minotaurs - which have huge range and take tons of hits to kill - into a claustrophobic corridor, where their attacks are practically unavoidable. The levels are all remarkably tedious, too, with the same tiles and layouts repeated over and over, and makes replaying them a drag. In other words, it's way worse than Konami's own titles.
The other major aspects ditched are all of the RPG elements. The enemies in the Metroidvanias were never particularly balanced, but they weren't really supposed to be, because your character would eventually get stronger, and amass an array of cool weapons to fight them with. At least there's a special secondary attack which calls out a partner for a quick and powerful strike, which drains ten crystals. You start off with Marisa, but then each of the defeated bosses will join you and lend you their powers. There are also some vague shoot-em-up elements - a tribute to its Touhou heritage, naturally - none of which really work.
Reimu doesn't have a double jump, but she can fly. This produces a bit of a quandry in certain areas, because if you can just fly over everything, then what's the point of fighting enemies? Well, fallen enemies drop little red crystals - the tougher the enemy, the more crystals. Reimu can shoot tiny little bullets on the ground by hitting Up and Attack, but it's her only weapon when zooming through the skies. It's remarkably weak, but it does fire rapidly.
The boss battles are all flying girls, most of whom have some kind of bullet hell-style projectile patterns. You can technically try to fight them with your whip, but you can also take to the skies to fight them in mid-air. But since your bullets are so weak, you really need to focus your stream on them if you want to do any damage. The flying itself brings up some concerns - you can't directions in mid-air without dropping to the ground first, and if you run out of crystals, you're basically defenseless without bringing the fight back down to earth. There's also a very serious problem with bullet hell-style games in 2D that apparently the developers never acknowledged - your sprite is much bigger, and without the full 360 degree of movement offered by overhead games, PLUS the addition of gravity, makes the bullet patterns all but impossible to avoid unless you just run really, really far away. MegaMari had the same problem. Developers! Guys! It's nice that you're trying to Do Something Different, but it just doesn't work, so cut it out.
There's some acknowledgment that the game is so poorly balanced, in that certain enemy attacks don't do very much damage. In most of the boss battles, it seems like rushing them with a partner attack, while doing everything you can to dodge the projectiles, is the most effective measure to kill most of them.
A lot of people got excited for this when it was first announced, because it does it look pretty good in action. But, at heart, it's still a doujin game, and thus expectations should be kept in check. Like most, it's a cool concept that just isn't executed terribly well.
At least they seem to be responding to complaints. There's already a patch that brings the game up to version 1.02a, which adds an intro movie with a vocal song, some cutscenes along with the excellent original artwork, a larger health bar, and the ability to restart on the last level you played after you exit the program, amongst other changes. Maybe with this kind of time and devotion, they can turn it into something better.