Having just finished Diary of a Wimpy Kid by Jeff Kinney, I thought I should add it right away.
Given that the book is half cartoon illustration and the rest of the text is set in a faux hand-written font, it's not nearly as long as it appears to be on the shelf. It takes about an hour of actual reading, tops.
It's a fun, breezy little read (as can be expected from the title and format of the book) and is rather obviously slated for kids, who are likely to enjoy it. (Hence it becoming a bestseller and spawning a series.) Reluctant readers are likely to pick it up, which is good.
My one complaint would be the unsurprising use of stereotypes which abound. There are times when the author seems to be trying to step outside of the done-to-death stereotypes (the main character admits that he wants to take Home Ec 2 because he was good at Home Ec 1, but won't because being able to sew doesn't help your popularity), but settles comfortably into them the rest of the time (the girls in class want to design a robot that will give dating advice and have ten types of lip gloss on its fingers). I guess the one redeeming factor is that the whole thing is being told through the filter of a grade seven loser, so maybe he's the one creating the stereotypes, not the author. Maybe?
The second book (Roderick Rules) is already in my possession, so I'm sure I'll take an hour to read that one, too, at some point.