I bought the first book in the series to read to my five year old son - I wanted something with chapters that would keep him entertained and coming back for more, and that is exactly what we got with these stories.
I didn't know anything about the books before we bought it - it was a chance find at Waterstones, but what a find!
My little boy is enthralled with them, we get an opportunity to discuss words he hasn't heard before, and he asks me whenever he can to read a chapter with him - I never say no.
I have to say that these books have the added bonus of giving us an opportunity to go to the bookshop together (sorry Amazon) to buy the next volume every couple of weeks. For me it is a great opportunity to get out of the house and have a little 'father/son' time.
Most of all, he is enjoying 'grown up' books, and he tells me that before we get to the end of the series, he will be able to read the books to me. Now, what more can you ask from a series of kids books than that?
Sure, the stories are formulaic, but I'm not a five year boy - to them, the stories are wonderful - I'd recommend these to any parent with a child aged between about 5 and 8, particularly boys, who will want to read them themselves, although the books do have a strong female character throughout, so the girls shouldn't feel too left out by these books.