It's okay. It doesn't make any mistakes and it improves towards the end. Halfway through, though, this didn't look like a show you could marathon and I probably wouldn't recommend it, unless you're a fan of the famous 2007 game it's based on.
It's a stylish (a bit of character in the inking!) death game in a subculture of graffiti artists, skateboards, etc. in Shibuya, Tokyo. Our hero is Neku, a surly teen who wears great big headphones to try to stop people from talking to him. Unfortunately, he's stuck in a parallel plane of existence called the Underground, which overlaps but doesn't intersect with the Realground of normal people. (Realground inhabitants are visible but intangible for Underground people and if you try to get in their way, they'll just walk harmlessly through you.)
The Underground also contains shinigami (reapers) who can conjure killer monsters from thin air and give you missions to complete within a set time limit. A countdown appears on your hand. If these ticking numbers disappear, you'll have completed your mission. If you fail, get killed or simply run out of time, though, you'll disintegrate.
The storyline's quite good, although you'll enjoy it more if you can forget how many episodes it has. The final climax on the final day... in ep.3? I don't think so. I imagine that worked better in the original game, as indeed would the game play itself. At least half of this show is Neku and his friends trying to achieve some easy, arbitrary task, on the shinigamis' orders. Imaginary monsters must be fought and so on. They appear from nothing and then disintegrate back into nothing when killed, which is rarely difficult. It's hard to convince yourself that they matter. In my opinion, a better adaptation would have reduced the number of game challenges and made each one harder and more meaningful... but the whole point of resurrecting a 14-year-old game like this would have been to appeal to the game's fans.
Well, up to a point. Apparently the game's monsters ("Noise") exist in two parallel universes simultaneously and can only be defeated by two players simultaneously fighting the same Noise in both of its zones. The anime doesn't try to reproduce that.
The cast are fine. Neku grows as a character, from his surly, anti-social beginnings. The plot can't be discussed without spoilers, which even includes basic information like "where are we?" "what's going on?" and "how did we get here?" I will, though, note that the entry fee requirement is harsh and the "only one" condition is an evil feature in a partner-based game.
Overall, it's okay. The last third or so is quite good. It sidelines its female characters a bit (well, the non-villainous ones) but the show's main problem, really, is that the game framework is a bit of a shackle and it's no coincidence that the show improves when Neku and his friends are no longer tied so rigidly to that daily bam-bam-bam. If you've played the game yourself, though, and fought through its challenges personally, you'll have a higher level of emotional investment and it's possible that this show might blow you away. (I'm speculating, yes, but based on an online review or two I've seen.)
I don't know if I'll rewatch it, but you could do worse.