You know what they say: bad books make good readers.*
They mostly toy with scariness rather than becoming flat-out terrifying, but each achieves a few genuinely creepy moments. Right now I'm reading #9, No Trick-or-Treating!, which as a "superscary superspecial" is just a tad longer than the first eight. That gives it an edge, because it can develop the setting a bit more thoroughly (a small country town which mysteriously doesn't celebrate Halloween) and let events unfold slowly enough to build up some real tension.
I'm a fully-functional adult (more or less), and I had to resist the temptation to peek at the last chapter to find out what happens. Ha. I would have loved these SO MUCH when I was the right age. I'm enjoying them enormously even now.