I just finished making an audiobook of this series for my boys to listen to while we're overseas. They read the entire series this past year, but I hadn't read through all the books till now. I'm still happy with the set after reading it, although I understand some of my boys' complaints about it better. They didn't like the book about China at all, and I was disappointed with it too. The writing isn't at all engaging in that book and the chapters are long. My older son who was in 5th grade and read like an average 5th grader struggled a little with the reading level (which is why I recorded it since he loves audiobooks). It's definitely middle school level. My then-3rd grader didn't have a problem though.
One thing I noticed more after reading through the entire series is that the Greek and Roman civilizations get a lot more coverage than other parts of the world. I'd have been happy with combining those two civilizations into one book, or expanding some of the others. The Persians in particular got the short end of the storytelling because they didn't quite fit into any one book perfectly, except possible the Near East book, and there just wasn't room in that book to do them justice. So most of what you learn about the Persians in this series is from a Greek perspective which isn't exactly unbiased.
But I certainly haven't changed my mind at all about thinking that this is an excellent series for middle school/logic stage ancient history. With the exception of the book on China, the writing is interesting, and in all the books the history is solid and detailed. I know my children learned a lot, and I learned more than I expected to.