The recent Power Girl trade was the # 1 selling trade for the month it came out and the girl has a distinct personality, and she is far removed from Supergirl's character in terms of traits.
She is popular and likeable, powerful, and she has a long history with tons of connections to several key elements of the DCU.
I can totally see her fending off threats from Bizarro, The Ultra Humanite, Mongul, Parasite, Maxima and of course she could get brand new villains and even her old rogues gallery could be revamped/revitalzed.
Give the book to a hot writer (Johns?) or a hot writer with a reputation to write solid female characters (Manhunter's Marc Andreyko) and a great artist like Rags MOrales, Dale Eaglesham, Amanda Conner or Terry Didson and have the tone of the book be a very action-packed, old-school superhero soap with dashes of humor and you have a hit in your hands.
how about making her a regular foil in supergirl's book, as is Kara is not all the interesting but put her with a brasher more powerful version of herself and you could have a good odd-couple type thing.
I'd rather see Karen in either her own book or in a book specifically for the Super Girls (her, Kara, and Cassie). It'd be like the Birds of Prey for the Superman family of books, but with Wonder Girl.
Either way, I agree, I find Power Girl much more interesting than the new Supergirl. Depending on how she were handled in Supergirl, though, as a co-star/mentor for Kara, I could see myself picking the book up again specifically for her.