Junie B. Jones is a character, and has a unique voice. However, she exercises a pet peeve of mine in that she "talks wrong." I had a lot of resistance to a character that continually coins words and makes grammatical errors (ones that a child that age should be overcoming), but then I loosened up and used the way she speaks as a launching point for discussions with my daughter on how something she said should be phrased, or could be expressed better.
So, that coupled with the fact that my daughter just tears through these books and wants to read them over and over adds up to a thumbs up for me. But do balance this out with books that are grammatically correct.