This is one of the best rated TV shows, and it is easy to see why. Animated or not, it doesn't matter. This show has a single story that builds slowly through the seasons to a dramatic finale. If you have a preteen, and want something to watch with them, this is what you are looking for. (Assuming you haven't seen it)

It isn't perfect, especially in season 1, there are some episodes that are a bit on the rougher side. I don't remember where it is exactly in the story, but the first season was definitely on the silly side. One episode is cute, especially for younger kids; but cringe-worthy for an adult to watch.

I get the feeling that they were having demographic troubles in the early episodes, because there is enough stuff that refers to super younger kids that it doesn't mesh well with the extremely mature and thoughtful side of the later seasons. It is also on the earlier side of the 'META' story concept, and they didn't add as much 'adult friendly' content as you will find in, say 'Gravity Falls'.

As the show predates Gravity Falls by about a decade, I think you can understand that this show was still cutting edge. If you want to go back further, try out 'Batman: The Animated Series' for a decade before Avatar. You can quickly see that producers were testing out the ability to field a more 'serious' carton, but were nervous that there wouldn't be enough 'silly' in it to capture the attention of the youngest audiences.

Still this show is well written, and follows KISS (Keep it Simple Stupid) and it pays out big. A simple, but powerful story, and they improved their pacing, not spoiled it. The best rated episode is the last, and it is truly worth watching the full show for. Adult or child should enjoy the full story.

Although, without a kid, I would strongly recommend episodes you feel are a bit too boring. The story isn't that deep.