We read them. Almost all of them.
The Good. The "Peek Into the Past" section in each book contained some nice bits of history. We got into discussions with each book about how the main character's culture constrained or liberated her, like Kaya's clothing allowed her to ride and run, but Felicity had to "borrow" boy's pants in order to be able to run freely. We also did contrast and comparison between the books - what were their houses like, how did they cook, what methods of transportation were there, what were their clothes like. The main characters do not suffer the loss of immediate family members, as I recall, just extended family.
The Bad. The books are, in DD's words, "formulaic and predictable". Ultimately, they end up being boring because every girl's story follows the same pattern. And, IHMO, the quality of writing suffers. They're also sometimes syrupy sweet.
We discovered the "Dear America" series and were MUCH happier with that. They are written as if they are diaries (but they're really fiction). The authors do not have to follow a particular set formula (as they did with the American Girl books), and so the books vary quite a bit more and are more interesting. However, they are also more real and gritty - the main characters sometimes have immediate family members that die, the atrocities of slavery are shown in a more harsh light than the AG books did, characters sometimes seem to have hopeless lives (but they turn out OK). The "Epilogue" in each book was good, especially at (usually) giving the story a happy ending.