My five year old son loves these stories, and I enjoy listening to them as well. The cameraderie among the siblings is refreshing and it is nice how resourceful the kids are. Each of the stories has an element of mystery to them, which keeps you interested. (My three year old, however, starts to get antsy with these stories, because they are so lengthy - we have to take Dr. Seuss and Nate the Great breaks for him!)
My son started listening to the Nate the Great audiobooks a few months back and quickly became bored with them. He loves that these A to Z Mysteries have a more involved story line and a bit more suited for "big kids". The language is appropriate (don't have to worry about hearing "shut up" or "stupid") and I like that the kids in the story are resourceful. The only complaint I have is that most of the stories I have listened to so far are about finding a thief...would be more interesting if they switched up the mystery a bit, but that's just me.
My 8yo son loved this and wanted me to listen with him. Very enjoyable. We now have another reason to visit Chicago so we can explore the Thorne rooms. He has requested the sequel.