tl;dr: Good middle grade mystery. Characters you'll like. Sammy is a girl.
I read this at the same time I was reading Lincoln Jones, and I couldn't understand how Sammy Keyes was so good and Lincoln Jones was so bad (that's a lie... I know why, it's just disappointing). I never read Sammy Keyes before because I thought Sammy was a boy. One of our children's librarians assured me that Sammy is a girl and that I had to read these books.
She was right (of course). I've only read the first so far, but it was really good. I liked the characters and the mystery kept me guessing. I won a copy at our Lincoln Jones book club, got it signed and passed it on to the eleven-year-old. No word yet on whether she liked it or not. I think if she reads it she will like it, but she might have moved on from mysteries.
Storytime: One time in the last hour of work (8p-9p) a kid came in looking for Sammy Keyes (for school I think?). I showed them the book and they were really thankful and super nice, but I couldn't stop wondering if the kid also had to read the book that night. They seemed really desperate.