There is surprisingly little of Supergirl in this book and nothing whatsoever regarding her supporting cast. The book opens with several pages of Lex Luthor plotting with a bunch of villains I've never seen before. After that, we get a brief explanation of the end of H'El On Earth that actually explains very little past "Kara got Kryptonite poisoning and is dying slowly." She's being looked after by a doctor we've never seen before. Then she gets into a fight with some lava-powered girl we've never seen before, who was apparently a villain in Superboy over a year ago. And apparently Kara's illness is having some kind of an effect on Karen Starr (a.k.a. Power Girl), who - you've guessed it has never been seen before in this book. In fact, they don't even identify her as Power Girl! So unless you've been reading World's Finest or are already familiar with how Power Girl and Supergirl are alternate-universe versions of the same person, you're going to be completely lost.
Way to help those new readers The New 52 revamp was meant to attract, DC Comics!
I can say little about the art by Robson Rocha. I only recall seeing Rocha's work once before and all I remember about it was relief that he would only be working on one issue of Demon Knights. What I see here does not impress me, particularly how ever single female character is posed for maximum provocation. Power Girl looks less like the powerful business professional she is meant to be and more like a businesswoman-themed stripper.
Supergirl #18 is everything that I feared this series would be when the New 52 began. Rather than standing on its' own merits, it assumes that you have been reading every other book involving all the other Superman characters and that your only interest in Supergirl lies in her connection to that legacy. The rich supporting cast created by Mike Johnson over the last 17 issues? Gone. The excellent artwork of Mahmud Asrar which didn't sexualize every single female character? Gone. Your favorite comic book critic? I'm gone too. I'm dropping this series immediately and advise the rest of you who were following it to do the same.