The manga starts about a year before the events of Persona 4. It's told from Yosuke's point of view, so it's an entirely new perspective. The story begins with Yosuke's move to Inaba (from the city), and chronicles his attempts to adjust to his new life.
If you've ever moved to a new school, you'd know that the period of time where you don't have any friends is
painful. The manga does a good job at showcasing that, and the general struggles of adjusting to a new life. Sure, things don't seem so bad for Yosuke on the surface; the people he meets are fairly nice (most of the time) and the scenery is great, but it's just not the same. Yosuke constantly goes through feelings of regret and loneliness, as he desperately longs to go back to the city. He ends up meeting a few a familiar faces (Chie, Yukiko, Kou and Daisuke), but he doesn't end up getting close to any of them. It's an interesting contrast from Yu, who instantly adjusted to his environment after moving to Inaba. To make matters worse, he pretty much loses contact with all of his old friends (with the exception of one... more on him later). Things change for the better when he meets Saki. Saki doesn't get much development in P4, and that doesn't change here, but nonetheless, her interactions with Yosuke are pretty great.
On the flipside, the manga really should have given us more exposure to Yosuke's life pre-P4... It eventually diverges into P4 territory, and while it's good to get another perspective on the established events, I really wish the author had stuck with the previous format, because it was much more interesting. There's also the matter Katsuragi, a friend from Yosuke's old posse. Yosuke still keeps in contact with him from time to time, and he's Yosuke last tie to the city but... you're left wondering why he was created in the first place. He gets decent exposure for the first half, but he doesn't have much of a presence for the most part, he's just... there. Unfortunately, his previous relationship with Yosuke was also barely elaborated on. Overall, he feels like a missed opportunity.
As far as Persona 4 spinoffs go, this is probably one of the better ones. It offers greater insight into Yosuke's character, and it's just nice to see him on his own. If you're a fan of Yosuke, you should give it a shot.