I would say absolutely take her. The time to do those sort of things is very fleeting, and at age 6, you are getting to the end of it, so take advantage of every minute and enjoy it while you can. We have only been to the Dallas location. We have done it twice and enjoyed it both times. At least the times we have been there, which the last time was probably 2 years ago, in the Dallas location they did not seat everyone at once. It was just like a regular small restaurant. The food was a little expensive, but not overly so (I think they charge about $15 a person regardless of your menu choice, but that does include an appetizer- if I remember right dessert may have been additional, but I could have be wrong about that and if I'm wrong, then the price really isn't expensive at all. I remember DD had a chocolate mousse flower pot and it was adorable.), but it was good and we felt like the portion size was fine. DD loved having her doll at the table and the doll getting its own plate and cup. Again, it has been a while so things could have changed, but when we ate there the cup and plate were china and we did not get to take them home, nor did we have a take home napkin ring/hair tie. It could be things have changed, or that could be dependent on location.
DD was hugely into AG from age 2 (thanks to seeing a catalog we got after buying something for our older niece) to 7. Most of her friends got really into it by age 4. At age 8, she doesn't play with her AG stuff nearly as much. She still likes it and does play with it some as she is big on dolls, but most of the girls in her grade at school and her other friends her age are not interested in AG much at all anymore. Enjoy it while you can! Those couple of AG lunches were some special times that we fondly remember and do not regret!
Yes, I agree that it would depend on location. Chicago, which is the one we go to, is a huge multilevel store and the food is expensive, just like everything else downtown. We got the hair tie, too. I can see how in smaller markets, pricing would be less expensive, stores smaller, and "extras" not available.