Personally, Power Girl is a dumb character. There, I said it. She is completely redundant and efforts to make her different from Supergirl are forgettable and just muddles up an already confusing character. I only think the reason they kept her around was so horny writers/artists could draw her boobs and tell boob jokes.
I always thought DC, at least post-Crisis, spent waaay too much time trying to "explain" where she came from when it would have been much, much, easier just to say that Zor El had two daughters.