About the best that can be said about this book is that it's not as objectionable as R.L. Stine's usual. Stine eschews the usual conventions of literature -- character development, plots that make sense, setting, writing technique, occasionally complete sentences -- in favor of stereotyped characters, bogus cliffhangers, and disgusting bits of violence that keep reluctant readers coming back for more. At least this doesn't have the rivers of gore that have been the highlights of some of his previous efforts, nor is it as scary.
The argument usually made in Stine's favor is that he gets reluctant readers, especially boys, to read, and this is true. As long as they graduate to better quality, that's a good thing. If your child hates to read but enjoys this, don't worry. Wait a year, and if he's still reading nothing but Stine, then you can worry.