I've installed that sga stock on my mossberg 500 coupple of weeks back. Couple of quick observations: It feels great in the hand (the angle of the quasi pistol grip is a perfect hybrid between pistol grip and traditional curved area of the stock), the LOP and cheek height are adjustable, the rubber pad on the end of the stock is very soft -- unlike the stone-hard I had on the original stock.
From what I've heard, the only downside to this stock is that one has to partially unscrew the stock, before removing the trigger group when disassembling the gun. This shouldn't be a huge problem, because the screw is much more conveniently located than the screw on the original stock.
I'll go out to test this awesome stock out on the range next week. But, so far, I'm really liking it.