I own 6 American Girl dolls, and only one of them is accessible to InnerstarU. The whole reason I got another AG doll was to be able to access InnerstarU, and I was not disappointed.
American Girl directs their products towards girls 8 and up, and this website definitely fills in those age requirements.
I know many people complain about this website because you have to purchase a doll to play on the website, but that's kind of the point. Other websites, such as Club Penguin, make you pay so you can go on their site, so there's no difference.
Overall, this is a great website to play on, IF you have an AG doll that can access it.