My 7-yr-old is dying to read this series because one of his friends is reading it, so I borrowed the book and checked it out - and it is worthless.
First, if the main character is in middle school then the content is clearly not intended for younger children. In this case, the funny parts (and I'll admit, there are a lot) stem from inappropriate behavior and a generally sarcastic/negative view of the world, which is not a good example for younger children (if anyone).
Second, there is really no academic value, since the writing/vocabulary is so basic that it is not even a challenge for my 2nd-grader, let alone for the middle schoolers who are presumably its target audience.
Third, while I have heard claims that it teaches life lessons, this is only true in the sense that children may infer lessons about proper behavior by seeing this as an example of what not to do. Unfortunately, many of the scenarios are so amusing that children (especially younger ones) will be more likely to laugh about it with their friends and mimic the bad behavior thinking that they are being clever or funny.
On the whole, there are too many good books out there for readers of all levels to waste time & money on this stuff.