I don't know, I was expecting something a bit more Artemis Fowl-like without the fantasy element. I expect it didn't help that I hit chapter 2 just after eating something that disagreed with me since it consists of several pages of detailed descriptions of collective vomiting! That's when I realized that Itch is really meant to be Horrid Henry for older kids.
I think the real problem for me is that Itch is dangerously stupid and I couldn't really empathize with him. Also, the obvious baddies were bad obviously. My daughter and I had a laugh round about chapter 5 or 6 analyzing all the foreshadowing of badness.
Anyway, it's not that i hated it, but I wasn't thrilled and she, being in the appropriate age group, liked it a tad better than I did, but only just.