This isn't a bad manga, but it's not particularly outstanding, either. It's a decent read if you're procrastinating about schoolwork or chores or if you need something for a long trip, I guess. It could be better, since it's chock full of clichés, like every other shounen romance I've ever seen, but again, it's not bad or anything. Just very average.
I wouldn't even bother to leave a comment,except that I can't get over the quality of the art. The backgrounds are amazing - they look absolutely wonderful. But the people! Everyone has the same face or hairstyle, with only very minor variations (maybe two extra locks of hair drawn in on one side, or the shading is different, or the face is slightly longer than wider), so it's difficult to tell the characters apart in many cases. For example, Yuu's sister Tsukasa, Mugi's childhood friend Manami, and his other childhood friend Sayuri all look pretty much identical at first. Mugi's exgirlfriend also has the same look. Sayuri is the most unique of the quartet for having visibly shorter hair, but the difference is really only noticeable if you're paying close attention or if she's in a panel with one of the others.
Also, for all the drawings of boobs and panties and so on, they're very poorly done. The anatomy is passingly good compared to some manga, but not if you consider how much time and attention these fanservice images receive. Boobs look like they're melting or are positioned poorly, legs don't line up with the hips properly, arms are disproportionate, and so on. It really bugs me when the camera angle is looking down a girl's shirt and the cleavage is completely unrealistic for the way the boobs are drawn, and this is almost every single time. The effect is pretty much that the artist hasn't studied anatomy drawing, and has instead just copied other drawings, or else isn't taking any care at all with it.
Also, this is a stylistic thing, but the way closed mouths are drawn is a bit uncanny valley. The lips are just plain weird, but I usually like the way artists with this kind of style do faces (particularly the eyes, though again, there's very very little variation except when variations are exaggerated to emphasize a particularly character type).
It's fairly easy to look past the wonky art to the story (though it still irritates me), but with the decidedly mediocre plot and the bad anatomy equal a less-than-great, if still readable, manga.