The premise of a yakuza trying to learn how sing karaoke from a middle school boy is funny and this story definitely is funny, but it has just the right amount of emotional conflict to take it beyond just a comedy short story. Just enough to make the boy's character a bit deeper, a bit more realist and the overall story a bit more touching without turning the story into a full drama.
The main characters are very likeable, one is the more relatable and the other the more charming, and their dynamic makes the story shine (and no, I'm not talking about the BL subtext here - for those interested/worried about it, the BL subtext exists in Karaoke Iko!, but it's relatively easy to ignore. It's in the sequel that the subtext is undeniably present). The other characters don't have many scenes to judge them individually, but none of them drags the story down. Instead they either help with keeping the story grounded in reality or keeping it light-hearted.
The humor is classic Wayama-sensei humor: not exaggerated, relying in things that aren't quite unrealistic, but outside of normal expectations, awkwardness, relatable thoughts and reactions, and great timing.
The art is pretty good, but the style is not quite like what we see in mainstream series. For me, it's a plus. It's more unique without needing a lot time to get used to it. The characters also look a bit more like an average person. It helps the story feel more real and relatable, and matches the humor style.
Overall, the story is quite solid. Good pacing, appropriate length and pretty memorable in my opinion. I recommend to those looking for a more understated humor and for a story that has a nice mix of drama and comedy.
And if you're wondering if you can skip this one and go straight to the sequel, don't skip. There's no reason to. It's just 2.5 chapters, it's easier to enjoy than the sequel and it will make what's happening in the sequel more understandable and meaningful.