I read this as a child, and the ONLY thing I actually could remember about it was the ending. Having recently reread the book...I honestly still can't remember what happened. None of the characters that the main characters meet matter too much except Elvis, who stands out because he's kind of quirky. Even Lucy, who's set up to be mysterious, isn't actually present often enough to be intriguing. Plotwise, nothing really makes sense about the things that happened. Two camp stories are given that are supposed to come into play as explanations, but they just add to the confusion.

I did like the creepiness about the camp, however. Never having been to a sleepaway camp myself as a kid, I always loved the camp stories and thought they were scary because of how the kids were pretty much surrounded by strangers who, as is wont to happen in a Goosebumps book, are not to be trusted. The campers themselves have horrific/painful things happen to them and don't react properly, and it does set a threatening and creepy tone. 

The twist at the end is great. You are given enough information to figure it out two seconds before the main character does, which provides an extra bit of shock.

All in all, I would recommend this book, even if it is a bit forgettable. It's creepy and silly, and fun to read.