I like them too, but lots of people don't. My ds1 (10) has read the first, but didn't bother with the others.
NB Nix's older books (Lirael, Sabriel etc.) are much darker. I haven't really censored ds1's reading for years, but even now I wouldn't encourage him to read Lirael yet, even though it is a superb book ... it's all about necromancy!
I really like Garth Nix, especially the Abhorsen trilogy. The Mister Monday series is much lighter - great for grown ups after a hard days work! I was really gutted to fin he hadn't written the final two books after racing through the first five. I thought about reading them aloud to my ds (not a keen reader, but enjoys being read to) but wanted to see if he ended the series well before starting on them (nothing worse than a disappointing end in my opinion).