We got this series based on a friend's recommendation and it was spot on. My son DOES love it, but I don't. It's trite and the writing isn't particularly good. I love the concept and theme, but the application just isn't a winner for me. The issues:
--Grammar and syntax aren't up to snuff. I know they are books for kids, but still.
--The kids never change. We're already 10 books in and by book 2 we know that Annie is impulsive and Jack is thoughtful. When Annie's choices get them in trouble, nothing is learned. She's just as rash the next time. And Jack says wait, let's think about this... but never does.
--Jack takes a bunch of notes about really mundane facts. Why can't he take notes about something useful to develop that researcher/scientist angle?
--Morgan le Fay is from Camelot but speaks as if she is from the 21st century.
--Everyone (even the Japanese and Innuit) speak English.
--It seems like Osborne tries to add a lot of drama to areas that don't need them ("Then everything was still. Absolutely still." Really? Again? Got it, it's still.) and then sort of speeds over areas that may actually be dramatic (like sharks and thin ice, etc).

In summary, it's not literature. Everything is glossed over: character development, sentence structure, climax building, plot intrigue, thought-provoking situations... It's no Roald Dahl or CS Lewis or JK Rowling or...

But it is for him, aged 6 1/2, and that's why we got them. He doesn't pick up on any of these annoyances. Unfortunately, we're reading them to him, so we have to make it through all 40-something, repetitive stories. Then we can get back to the good stuff.