I'll keep this short because I can't pick out anything glaringly wrong with the book. Power Girl and Supergirl together is amazing. The book flows very well and Mike Johnson does a terrific job. Power Girl get's the old costume back and it looks great, I think this book will really help Worlds' Finest as well. IT will be nice to see Supergirl interact with them in that book as well.
IF you haven't read the book please do. Pick it up, its well worth you're time and your money.