I'm basing these reviews off our system at the Kinyn Chronicles blog.
Ridiculously simplified summary: 2 girls from different worlds become unlikely friends and face a yellow fever epidemic in 1853 New Orleans.
Christian content: (not rated) The American Girl books are not Christian books and I don't expect them to be such. The main characters do go to church and pray together in times of turmoil. A very ill character imagines she'll be an angel when she dies, which isn't really a Biblical viewpoint. A character says "mon Dieu" which the book translated as "good heavens" (though the glossary also notes the more literal translation "My God!").
50 possible points; in each category, 6/10 (or 3/5) would constitute 3 stars, or average.
Compelling - 8 points (out of 10). I had a hard time getting into the second book, Meet Cécile--it was one of those books that just didn't flow for me immediately for some reason. Book 3, where the fever picks up (along with some unlikely elements) is where things get going for me. (From the American Girl site it sounds like some kids (?) would disagree.)
Characters - 8 points (out of 10). I didn't expect to like Marie-Grace, but she has several moments where she is genuinely shy. I was incredibly shy as a child (my dad called it "painfully shy") and still have a lot of those traits. I adore seeing shy and introverted characters in fiction, though I don't see many of them. Cécile is the exact opposite--rather bubbly and outgoing. She won me over in Marie-Grace's book (not written by the author of Cécile's books) but I didn't feel quite as drawn to her as I did to Marie-Grace, who has the advantage of being the newcomer to New Orleans (and thus the one that's a bit more relate-able). A few supporting characters stood out for me, particularly Ellen (Cécile's Irish maid) and Marie-Grace's father (doctor), but I didn't think many of them were very memorable. The books are really about the two main girls, though.
Writing/editing - 7 points (out of 10). I didn't find any typos or anything, but the writing had a lot of things that writing groups will drill out of you. The writing is few of adverbs, "was"es, telling phrases like "Maman looked stern", and such. (If you ever reread the old American Girl books--and even the Rebecca books--those sorts of issues are common throughout the series.) It did take a little while to get used to the style but once I did it wasn't too distracting.
Plausibility/believability - 3 points (out of 5). The historical aspects felt more or less "right" to me, but I'm honestly not sure how much interaction a black and white girl would have actually had (though as a modern girl I love to see it!). Both main characters had some special talent I thought rather implausible--Marie-Grace has an outstanding singing voice, and Cécile (like the last historical, Rebecca) dreams of being an actress. (Unlike Rebecca's 1914 parents, Cécile's parents don't seem expressly mortified by this.) I feel like this was sort of a fantasy of 1850s life more than a realistic portrayal, though again, that's not terribly uncommon for the series.
Positive - 3 points (out of 5). I like books with happy endings/that make me feel hopeful. These have a mostly happy ending. Sadder than a typical children's book, but much more optimistic than an adult historical novel.
Gut reaction - 3 points (out of 5). I really enjoyed these books overall, even if they are perhaps sort of escapist, so I'd like to say my gut reaction is that I liked them. Unfortunately, some of the art is just stiff and awkward-looking, and art has always been a big part of these books. The tiny sketches in the books are not nearly as high-quality (or useful!) as they were in the other books I remember. (Others complain that the books don't use the same structure as the original books--Meet Kirsten, Kirsten Learns a Lesson, etc. As much as I liked the idea of comparing different holidays and milestones across time, they changed that a few books ago so I'm used to it by now.)
Bonus points to balance reviews - (5 points)
Total points: 37, or 3.7 stars