It's a brave choice, following up Potter with something in the "general fiction" or even possibly "chick lit" category. Sort of the equivalent of Radcliffe following it up with his movie about Allen Ginsberg rather than signing up for Star Trek and Emma Watson doing a cameo in that Marilyn Monroe movie rather than signing on to Hunger Games.
Thing is Rowling is set for life. She doesn't have to write another word and she'll still be richer than most kings. So this gives her the opportunity to experiment. And who knows - you know full well this book is probably already lined up for a movie even before it sees print. Maybe this one will get Best Picture of 2014 or something. So best of luck to her and if the book appeals to me when it's actually released I might give it a try.
I just hope she gets that Potter Encyclopedia written up (which has been described as the Potter equivalent of the Silmarillion).