Under Night In-Birth definitely fits into the category of not many people hearing about it and even fewer actually playing it.
The UpsideProbably its best appeal is how simplistic the game is yet at the same time is deep. While only 4 buttons with not too many options that are any different from something like say Guilty Gear or blazblue. I find this game is just plain out fun.
Despite not having the manual (I got this through Gamefly) I quickly picked up how this game worked in the span of 2 hours. Only 4 buttons but you quickly discover how the game is supposed to work and what to watch out for. My favorite part of this system is that there's no real punishment for playing too passive or too aggressively. As long as you play it properly and not like other games like say UMvC3 where you can play run away and get credit for it. Or Guilty Gear where aggressive ends the match in less than 15 seconds. What I'm getting at is. If you're just running away and shooting projectiles you'll only get the max benefit if you're actually hitting your opponent. If your opponent blocks then the advantage can easily shift to there favor. Same can be said for playing aggressive unless you're actually hitting your opponent, if they block and even counter your moves then the advantage shifts to his or her favor.
I like the look of the characters as well. There's nothing to really complain about in there design and there's nothing that makes them stand out screaming notice me sempai. You can like or hate the characters as you wish. Even there personalities while somewhat anime tropes at least they're not focusing so much silly stuff like, "oh this character has boobs and I'm jealous." or giving focus on them by giving them strange fetishes. I don't mind that stuff just it seems to be the common point of fighting game characters now.
The DownsideThere's no one online playing this game. Even fewer that play this offline. At first when this came out like Blazblue and many other titles everyone that was playing this game generally imported it. Thus got the game real early compared to the US release date. There weren't that many playing then and there's fewer players on it now. I went online about 3 times and could rarely find a ranked match where. In player match rooms there's very ppl playing at a time. If there was fans of this game, they're likely gone to other games now. This is one of those games you'll have to hunt down players and ask them to play at a specific time to get matches. In other words good luck finding people to play.
Arcade mode honestly should just be named Story mode. Whenever you start arcade mode unless you're one of those people that skip dialogue and just want to fight expect to sit in your seat for about 5 to 10 minutes listening to the characters monologue before a fight. While the story is interesting I find it rather annoying to see this in fighting games. Its saving grace aside from being able to skip it is that it only happens at certain stages of arcade mode (unless you're playing Hilda who apparently has something to say to everyone). I honestly would rather have this like Street Fighter Alpha where the conversations are short and you don't have to listen to it again if you lose. The game does leave you with endings that leave it up to the player to determine what actually happened.
While I hate to admit this since I hate tier list. But the way characters fight. There's only 2 classes. Good or bad characters. While all characters can win the bottom half have to really work in order to actually get a win. In other words god tier or shit tier take your pick. All characters are enjoyable to play as but you'll quickly notice how hard it is to play as certain characters and others only have to press a few buttons.