Solid 9/10. It starts out slow and boring, and kind of confusing (like why the f#$% can't I put more than one card down!?), but I decided to give the game the benefit of the doubt, because I know Naka is an intelligent guy, and I wanted to see his vision unfold.
...7 hours and 150 cards later and I'm on the 60th floor of the dungeon.
This game is HARD. And it gets very grindy after you break into the 40th floor. Very grindy. I don't think it's right to give my summoner only +1 EXP for killing a monster that one-shotted 30+ cards of mine. Such a powerful monster demands a much greater reward, and it's out of proportion with the previous floors, which built up experience fairly quickly. It took me most of those 7 hours just to break the 60th floor, and when I finally did, I had to sneak past two "overwhelming forces" in a row on floors 58 and 59; it's like the game literally didn't want me to get to floor 60. That was ridiculous. Like, if you haven't fought an overwhelming force (they're orange instead of white) yet, don't even bother. Your only hope is to stealth your way around them. If you get in a fight with them, you're dead. Try to run, they'll one-shot you even at full health. Try to fight, and you'll exhaust your entire deck before you've even taken off a quarter of its health. They're impossible to beat unless you're WAY over-leveled for the floor you're on. Like, if you can break floor 60, then you can probably beat the rare overwhelming force enemy on the first 20 floors, and you might burn out half of your stock just doing THAT. That's how OP they are. So naturally, when you're going for a floor break, seeing the message "You sense an overwhelming force" is going to make you utter the word "shit", because you know that you only have one chance to MAYBE stealth your way past them, and if you get caught, you're fucked. One of my failed break-60 attempts ended because an overwhelming force spawned RIGHT NEXT TO ME.
What sucks the most (more than getting spawn camped by the most OP enemy in the game) is when you get Pac-man'd, i.e. fuckin' inky and blinky decide it'll be funny to corner you and gangrape you, one-shotting two cards every turn, and since you only draw one card at a time (instead of replenishing your hand to 5 cards regardless of your current card count, like a normal card game--remember that yugioh episode where yugi lost because he ran out of cards due to a peculiar rule regarding post-turn drawing? Neither do I), that means that two monsters cornering not only disallows you from stepping back to strategically take a hit in place of your cards, but you get to watch your field continually shrink as the game's broken mechanics and overpowered monsters fuck you in the ass until you're out of cards on the field and they two-shot your summoner (this is only further exacerbated if you're unfortunate to run into an excess of non-monster cards). Normally, it's a simple race against time to kill one of them before they overwhelm you, but on the higher floors, you might as well just give up, because it takes a million hits to kill one enemy. Died more times to that than anything else. Needs some serious re-balancing.
And the inventory management system is shit. It's just shit. Look, man, if you have 100+ cards, clicking those arrows to scroll becomes a huge pain in the ass pretty quickly. I end up spending 20+ minutes powering up cards and adding new cards to the deck just because I have to sit there clicking that scroll arrow for a million years. There are a lot of simple improvements that could make it much easier. Keyboard controls, page up/down buttons to scroll 9 items at a time, the ability to swap cards from your inventory into your deck en-masse instead of having to click every single card followed by the swap button individually, and the ability to "lock" the left-clicked card in the power-up menu, so I don't have to go back and forth to nearly the top and nearly the bottom over and over again, which is EXCRUCIATING and reminds me of Earthbound's clunky item shop UI. Those are four things that would significantly improve inventory management. and I REALLY think it's necessary, because the current system doesn't scale well at all. I'm having this much trouble managing a little over a hundred cards. Imagine how horrible it'd be to manage a thousand cards. Maybe I want to REALLY stock up to raid a dungeon and force my way to the next 20 floors through overwhelming numbers (seeing as they'll just one-shot my cards anyway, unless they have 500+ HP). Not a viable option with the current inventory management system.
Also, the music is extremely loud and not that good. The second part is highly subjective, yes, but I ended up muting the tab after about half an hour because the loop was getting annoying. But above all, the volume needs to be fixed.
And although the artstyle has this "deviantart" look to it, it's actually kinda charming, and grows on you after a while.
Aside from that, the game is surprisingly fun. I usually hate card games in the vein of hearthstone/yugioh, but the fact that I can look at all of those flaws, and rant this much, and still give this a 9, pretty much sums it up.
Fix those issues and buddy, you've got yourself a spot on my mental "favorite games" list.