One thing must be made apparent: this work was put on hiatus for many years as Bisco Hatori-sensei worked on Ouran High-School Host Club. As such, the style of art and storytelling is split between two different parts.
The first part is a decidedly 90s style. The style Hatori-sensei uses at this point looks especially hand-drawn without looking like a rough sketch, and so is very appealing.
Her style of writing at this point is incredibly comedic, never really spending too much time on darker elements. Even so, this doesn't detract from payoff of the story; characters still feel strong and the messages don't feel tacked on at all.
The second part is very obviously post-2010, with very smooth-looking art and more laid-back characterization. Things are much less exaggerated, and it makes for an excellent look.
The characters certainly seem to have grown up in the year that has passed over the real-life hiatus. Not only a creative use of such a story gap, but also a good way of explaining the change in general tone as well as giving a reason for winter to have come once again. The story becomes much more serious by the end; the series may have started out light-hearted, but it is hard to call it a comedy in the strictest sense.
Of course, Hatori-sensei has a way of infusing energy and humor into everything she writes, and this is no different. An excellent starting point and sign of good things to come all in one sequence.