We own several Junie B. Jones books, and my 8 year old daughter and I enjoy the stories. However, I always read the stories TO my daughter, correcting the poor grammar along the way. The books are written this way to give an air of authenticity to them, in that they are being narrated by a kindergardener. But how many 6 year-olds do you know who say things like "I quick runned fast," or refer to their mother as "That Woman?"
I do recommend these books to read to your kids, but definitely not for beginning readers to read on their own.