Kids on the Slope is a coming-of-age high school story, based around the friendship of two young men and their shared love of music. I decided to check it out largely because it was directed by Shinichirō Watanabe (Cowboy Bebop, Samurai Champloo, Macross Plus), and I generally like music themes.
The music is often used as a means to bring the characters together, which is necessary because they're constantly finding things to be upset about. Both Kaoru and Sentaro have tempers, and they explode often. Kaoru in particular often says things he regrets almost instantly, but has a hard time taking back...quite a few "open mouth, insert foot" moments. They're arguing, or sulking about a prior argument, a good portion of the time. And both have issues, mostly revolving around family, that drive them away from each other and friends in general. Music brings them back together repeatedly.
Inasmuch as Kids on the Slope has an ongoing plot, it revolves around a couple of love triangles. Between Kaoru-Ritsuko-Sentaro and Sentaro-Yukina (an artist girl at the school)-Jun, there's plenty of angst to go around. Misunderstandings and confusion abound, in large part because no one is willing to come right out and tell anyone else how they feel. Seems pretty accurate from what I remember of high school. But really, the romance plot isn't the point; the whole series is more about friendship than romantic love.
All of this could easily turn into either a really good story with interesting characters, or a horribly sappy string of cliches with no depth. Fortunately, Kids on the Slope is the former. It works because the characters are well-rounded with solid personalities, interesting backstories, and an assortment of flaws. You won't find cardboard stereotypes here.
From a production standpoint, I found nothing to dislike about Kids on the Slope. The artwork is well done and avoids the big-eyes-and-skimpy-outfits themes that make it difficult to take so many anime shows seriously. Much of the music is great, and while there are some recurring themes, it doesn't fall into the "play the same song over and over" trap that afflicts a lot of music-themed series. Having the characters play covers rather than original music helps with that, I think. And the series isn't very long...twelve half-hour episodes...so it doesn't drag on and nothing feels like filler.
Kids on the Slope is a solid series that just about anyone should enjoy. The friends-making-music-together theme made it particularly fun for me, I think, since I did a decent amount of that myself in high school. Did a decent amount of the foot-in-mouth stuff, too. But even if those bits of nostalgia don't apply to you, you'd likely enjoy it simply as a good story about friendship.