DD got her first AG doll -- a Just Like Me doll -- for her 6th birthday. For her 7th birthday she got Mia. For her 8th birthday she got Julie. This year, for her 9th birthday, she wants another look-alike (I think they're called My American Girl dolls now).
I really like them. I wish I'd had some when I was a girl. I'd have adored them. They're very well made, IMO, and they hold their value. If you decided to sell them later on you'd probably get back at least half what you paid, possibly more. We don't plan to ever sell ours. When DD outgrows them I'll hang onto them for my grandkids.
As for what age is appropriate, I'd say it depends on the child. DD was very responsible and took good care of her toys, so she got her first doll at age 6. She plays with her dolls often, at least once a week -- getting out all the clothes and accessories, arranging tea parties and sleepovers, etc. They still look almost new. A few of the clothing items have gotten holes (some things are better made than others), but all the furniture and food accessories are still like new.