The 39 Clues are good. The Missing series is actually written by the author of Book 10, and I was really impressed by the author of Book 9 as well. They are better than Septimus Heap, meaning I am actually motivated to read past books 1 and 2. Some of the writers are a little iffy, but I think overall it's a really good effort.
It reminds me of Percy Jackson in some instances (it's more than just Riordan this time), granted they are both meant for the same age range. I think The 39 Clues can get a little bit darker at times, not in the "world is coming to an end if we don't save it," more the issues it looks at. I sometimes wish they had developed some of these issues more, but I can see how that would be too much for a young audience.
All that being said, I look forward to the release of Book 11