Just managed my first complete playthrough (as opposed to dying partway), and I have to say, it has a lot of potential. My only complaint is that it's REALLY thin on powerups and content outside of the +Stats/-Stats double edged sword upgrades or straight +stat upgrades. The only ones of any note are the secret ones you can get from side rooms where you meet a hologram (basically the Dr Light pods from Megaman X) that offer you extreme modifiers like "huge +attack, no longer able to charge shots" and the actual slotted gear (helm/chest/boots) which offer themed functions like hovering or knockback immunity.
Nina (X) and Ace (Zero) have a default weapon, with 2 alternate weapons they can pick up from loot chests that change how they play. Nina gets a gun that shoots in front of her and at 45 degrees up and down from that, or a gun that shoots IN ALL 4 CARDINAL DIRECTIONS. Ace meanwhile gets his saber shortened for rapid fire stabbiness (which is useless on bosses because they have invincibility frames after each hit) and a polearm that lets him attack in...say it with me this time...ALL DIRECTIONS whenever he's jumping. There is no rarity difference on these either, I got the permanent upgrade that puts a weapon chest at the start of a run, and I realized almost immediately that it's in your best interest to just restart if it's not the omnidirectional weapon, because 80% of this game's challenge is hitting enemies on your vertical axis while platforming. The special "boss" weapons are almost universally terrible, with the exception of Ace getting a chance to use a ranged weapon and both of them being able to use the fire shield that blocks projectiles.
Decent minimalist story too. When you die you can find your old corpse on the next playthrough, which drops a powerup. The text when you salvage it says things like "don't think about it too much", hinting that it's intentionally morbid...
The scientists running the thing are basically pulling a GLADOS on you. They continually make copies of Ace and Nina with the intention of sacrificing them against their boss monsters to optimize THEM, not YOU. In the final fight, you fight dozens of fucked up versions of Ace and Nina as trash mobs and the boss characters use upgraded versions of all the boss weapons, including the ones that aren't obtainable by Ace and Nina.
I wouldn't pay $15 for it, but if they release more content for it or put it on sale, they have my attention. Until then...Yar.