So I finally saw this (bought the DVD for my wife's birthday and watched it as a family over the weekend)
Its tough to compare it to a classic Disney princes movie, like Frozen or Beauty and the Beast. Its more on par with other pixar "also" movies, like a toy story 3 or maybe Up
With that in mind though, I'd give it 8/10 as a watchable children's movie. The characters are likable and the plot is uniquely original. I think it does a good job of covering it's complex sci-fi concept in a way that makes sense and helps the plot along, without confusing young kids.
I also like the lesson it teaches and that, while Joy was the main character, we learn the importance of accepting sadness.