I've been watching State of Grace since it first came on, and I can safely say that it's one of my very favorite shows.
All of the actors seem fit for the parts. You'd expect that modern twelve-year-olds would not be able to act well in a 1960s setting, but Mae Whitman and Alia Shawkat do a great job. All of the characters are very well cast.
Another thing is that they seem like normal people. Grace and Hannah worry about normal things that typical seventh graders do. They don't try to behave beyond their years or sit around and gossip about boys. Neither of them seem stuck up, and they're people that normal kids would like to know.
The show has also improved a lot in the past year. Some episodes were kind of boring in the first season, but now I wouldn't miss one! There is a lot better of a balance concerning air time between the Rayburns and the McKees. In the first season, the plot would usually focus on one of the families, but it's so much more interesting now that they're both featured in every episode. It makes it a lot more fun to watch.
I'd give this show a 10/10.