There's many reasons to buy this book...
...but from someone that hasn't read one issue of BATGIRL since the whole NEW 52 started, not many of them involve the story. First page in I felt a little like I just walked in on my best friend in our dorm room. Not that that ever happened, mind you, but the analogy was sound.
From what I can gather, BATGIRL has just been severely injured battling a character called KNIGHTFALL, who, like many super-villains, reveal her insidious plot in great detail just before BATGIRL hands her her butt. From there on out, it's not much of what you thought you'd see in the book with the exception of an ominous phone call that no doubt leads readers into the REAL reason this book is my pick.
BATGIRL #13 is solicitated as a PROLOGUE to the highly anticipated NEW 52 BATFAMILY crossover "DEATH IN THE FAMILY". It's also the "re-introduction" of THE JOKER into the NEW 52 Universe. It sports a die-cut cover with the Joker's face and matches the companion release BATMAN #13...a book that I admit I haven't yet read as I write this to you. It's written by GAIL SIMONE and drawn by ED BENES...so it's not lacking talent at all. I also had to travel to 4 different comic shops to find one copy of this issue...meaning either A) fans dig it. B) Speculators are buying it or C) Noone ordered it. So again I say, there's many reasons to pick up this book. I foresee this possibly going as high as $10-$20 by the weekend at this rate on sites like FEEBAY. Hang onto your copy for another couple weeks and perhaps you could be looking at a $20-$40 book.
Either way, it'll be a $2.99 well spent.