I might be biased here :) but I've been reading Ally Carter for a few years already and can't help but love the direction the books are taking.
The series started off with this humorous mischievous feel to them, and as Cammie, Macey, Bex and Liz grew, so as the undertones of books changed bit by bit.
Last couple of books are getting darker, and Gallagher Girls are slowly starting to understand that being a spy is not a one big adventure. It's dangerous, heartbreaking, it's a burden and responsibility.
Now that The Circle, a mysterious terrorist organization solely consisting of double agents all over the world, will do anything to kidnap Cammie, she doesn't know who is her friend and who is the enemy (because The Circle recruits very young) her whole world crumbles and she has to choose who to believe and take her destiny in her own hands.
I love Cammie! I can see the potential in her. She will be glorious when she grows up, and I really want the series to continue and grow with her. The ending in this book is bittersweet, and I can't wait to see what's next.
As I said I'm biased, I adore spy stories, and Gallagher Girls is something between Stormbreaker and St.Trinian altogether, but it's also a love-or-hate book. So, decide for yourself which side you're on.