Dave is pretty correct here. There have been TONS of threads (literally, thousands of lbs. worth of electrons...do you have any idea how many electrons are in a ton? Well, it's a lot, let me tell you. More than five, anyway...
) on PGO or "Pistol Grip Only" shotguns and what, if anything, they are good for.
Unfortunately, we usually end up in a hot blooded argument between those who find them useful for defensive purposes and the rest of us. Wait, see, even
I can't discuss them in a neutral way, and that's saying something!
They can be fun in a goofy challenge type of way. (We used to shoot skeet with them. Or, rather shoot AT skeet with them.) They are not painful to shoot and won't hurt you if you use anything like good practices to shoot them ...though folks do seem to be able to find ways to avoid good practices and manage to hurt themselves anyway.
From a practical standpoint, they are a bad choice that folks seem to desire with surprising intensity to make, despite all the evidence and proof of the fact that they make you slower and far less accurate than a full-stocked shotgun. In my way of looking at things, slower and far less accurate are serious negatives to a defense-of-life tool. Especially when the benefits are nonexistent or at least unimportant.
Anyway, the first thing you need to do is read this:
http://www.thehighroad.org/showthread.php?t=44465
That's Dave McCracken's PGO and hip-shooting thread, and it is absolutely the primary treatise on the subject from someone who has spent more time with one in their hands -- and instructing others on their use -- than any other member of this forum, and possibly anywhere else.