I remember when my mom bought me the first book in the 39 Clues series. I was already in the upper part of the target age range, and I'd read many mundane kids' books lately, so I admit, I wasn't expecting much. (I hadn't read anything by Rick Riordan before, or I would've known better.) I was wrong.
The 39 Clues is, in my opinion, one of the best children's series ever. It doesn't "talk down" to readers; it's full of true-to-life perils (and though I feel sorry for the characters themselves, I appreciate the way it shows psychological repercussions the series' escapades have on the kids). It's full of adventure, suspense, humor, and heart. The Cahill world is engrossing. And it's educational (yet I was so caught up in the action, I didn't realize that on my first read). There's character depth and development that people wouldn't expect from kids' books. Villains are real threats, and the morality is more complicated that just "black and white". And the premise is awesome.
I'm an adult now, and I still get each book the day it comes out. And I'm not the only one in my family who enjoys the books. There's something about the 39 Clues that appeals to you, whether you're a child, an adult, or somewhere in-between.
I'll admit, the series has a problems. The first series was notably better than the second, and many fans complain about the changes made to characters' personalities (though some changes are realistic and understandable, at least) and the series' main pairings. (Amy/Ian is, in a word, "lovely," and I miss it, too, guys.)
But overall, the 39 Clues still charms, and I'll be along for the ride until the end.
Edit: I love the series, but I was sorely disappointed with the second series' ending, and I wasn't alone. Many fans have complained about the decreased quality of writing, especially regarding many beloved characters behaving egregiously out-of-character. It's sad when the main complaint is about the series' heroine, usually caring and smart, acting like a brainless jerk. It's equally sad when fanfiction writers portray characters more accurately than some of the new authors. Even so, I'll stay tuned for the third series... and pray the writers take note and remedy these issues for the sake of their adoring fandom.
Edit: So far, Series Three looks promising.