Power Girl never had an easy go of it. Starting off as an after market Earth 2 Supergirl stand-in, she spent her appearances in the 70s angrily trying to stand out on her own merits and defending her bustline. She spent the 80s and 90s dealing with constantly shifting origin stories, being, quite frankly, pretty shrewish, and also defending her bustline. In the aughts, though, I think they finally got her right. Rather than focus on what she wasn't, writers finally (and brilliantly) helped her find true strength, personality, and the wherewithal to to turn her life from tragedy into triumph. Still lots of jokes about the cleavage, but she seems to laugh it off a lot more easily now.
Of course, a lot of the improvement comes from the art of Amanda Conner, who can express more personality on the page with a single facial expression than the most flowery words of even some of the best writers, and she's worked with some good ones while drawing Power Girl. The 4 parter with Geoff Johns from JSA Classified was fun, if heavy on the continuity (shocking, I know). The first 12 issues of her ongoing, though, written by Jimmy Palmiotti and Justin Gray, have fun, action-packed plots, and are probably some of the most charming and endearing comics written in the last 20 years. If you haven't read those issues, go and get the trades. If you have anything resembling a soul, you'll very likely enjoy them. If Palmiotti, Gray, and Conner wanted to work on the Power Girl book forever, I'd read it forever.