Read all five books during a snow day last week, and rather liked them. Warning: The first book is out of print, and people keep losing my copy of Bad Girls, so I buy one whenever I see it!
Bad Girls: Mikey and Margolo both have their own problems and personality quirks; they aren't bad, just normal 5th graders who get involved in normal, silly problems and scrapes. The first book is fun because almost all the action takes place in the classroom. I felt sorry for the teacher! I didn't feel quite as sorry for Mikey, the "badder" of the two girls; she verged slightly on the obnoxious. Of note: my reluctant 5th grader LOVED this one.
Bad, Badder, Baddest: This takes place mainly at Mikey's house, dealing with her parents, who are divorcing. Margalo's house is chaotic as well, with siblings, half-siblings and stepsiblings, but they don't seem to go there as much.
It's Not Easy Being Bad takes the girls into 7th grade and deals with all of the challenges of fitting into middle school.
Bad Girls in Love is 8th grade and deals with the somewhat more complex social issues faced when boys and girls no longer hate each other.
Bad Girls, Bad Girls, Whatcha Gonna Do focuses more on the variety of activities that students get involved with in high school, as well as all of the social interation.
Bad descriptions, I know, but I did enjoy the series and how it followed the difference developmental levels of children as they go through school. Well worth a look if you have it on your shelves.