First off, this book seems to be for an older crowd than the Clarice Bean readers. Younger kids might read this and enjoy it, but a lot of the humor and style seems to be aimed a little higher.
Ruby is a remarkably strong and well-developed character. There is a certain amount of that 13 year old snark that is more or less required in this type of book, but Ruby's 13-ness has some personality and originality, and is not at all just standard issue teen kvetching. It's hard to have a teen heroine who is a wiseacre and yet still is engaging, but Child pulls off this balancing act. Right there the book won me over. Even when there is the occasional false note, (sometimes Ruby can sound too adult or a little too smart mouth), the book immediately breezes on to a more appealing scene.
Parents are standard issue lame, but Lauren Child makes them so hilariously and disastrously clueless that they actually become engaging characters in their own right, instead of just annoying afterthoughts. (Think of Bertie Wooster types as parents.)
Ruby's sidekick, Clancy Crew, (Nancy Drew?), is funny and sharp and loyal, and so works as an interesting second banana, not just a dopey foil.
The whole framing story - that Ruby is a super observant, super dedicated detective/code breaker - sets up nicely and works just fine. There's espionage and cryptograms and close calls and escapes and villains, but it's not the plot, it's the personable and engaging Ruby, that keeps the book going.
So, here's my bottom line. Everything in this book is just a bit, (or a lot), better than the usual. There are sarcastic observations about school, but more perceptive and funnier than usual. There are amusing thoughts about friends, and clothes, and parents, but more acute and engaging. It's all just sharper, hipper, more distinctive, more creative, and more original than other similarly targeted books. Seems to me that's about the best you could ask for.