Ok, I'll say up front that these are great stories for kids to read. I've been reading them to my 4 year old for some time, and the pace, characters, and action are all perfectly suited. Each of these books grabs his interest and makes him excited about reading more.
The problem I've got with the series is the author's skillset with the language. Some folks may want to overlook this based on the content alone; I feel this view is simple and potentially detrimental to the child. For lots of kids these are first readers, and help develop a child's view of language structure. They're going to start off mimicking the writing 'style' the see; in the case of these books, they'll be seeing a lot of poorly defined (or non-defined) paragraphs, sentence fragments, and grammatical errors. They'll be seeing poorly written examples that will help establish the basis of bad writing habits which may take a lot of work to erase. Whoever edited these books needs a new career.
That said, we skip around the series quite a bit. The latest books are comparatively excellent as far as language mechanics. There's a definite progression in writing *quality* throughout the series, and content is high for all. I just wish Pope-Osborne had taken a creative writing class or two in the beginning.
If you're reading these books to/with your child, you can do some on-the-fly editing. If your child has enough language skill to spot the mistakes, they'll be fine. Otherwise, you might want to proceed with a little caution, or skip ahead in the series.