First off, what the hell are you doing looking at AR's - You pinkie Commie Rifle lovers aren't allowed across the Line in the Sand.
Okay, so here's the deal. Sig used to make a military model called the 550. Depending on who you talk too, it was either a great attempt, with a budget cutback on execution that lead to it's downfall, or it was the wrong product at the wrong time. Either way, it kind of went by the way side...
Now the 556 Classic is the same basic design, with some new upscale innovations.'
It's a gas piston driven system, like your AK's, but it has a two position gas valve for timing ( suppressed fire anyone? ).
The entire rifle, minus receiver, is polymer on the outside, so it's more durable than plastic and it won't scratch up as easy. The receiver used to be steel, which made it a bit heavy, but on the plus side it was STEEL and not aluminum.
On the down side, for me, is the price ( Expect them to go for north of $1800 with a quick Google search ) and they almsot all have a 1:7 cold hammer forged barrel. Hammer forged barrels are alright, but they aren't the best for accuracy - add in the piston system and you have two negatives in teh accuracy department.
Now, 1:7 is not the best choice for the available bullet weights in common "retail" land....
1:7 is a faster spinning bullet rate, which means you have to shoot the heavier bullets out of them ( probably in the 77 grain, or higher, environment ). Most of the common civilian and military stuff is either 55 grain or 62 grain. With a faster twist rate, and the right conditions like high heat and low moisture, you could very easily overspin a 55 grain bullet and get it to particl-ize ( I made that up ) and therefor would be useless in the field.
Now, if you were reloading, the 1:7 twist with the heavy bullets would give you great knock down potential over someone shooting the 55 grainers all day long.
For Minute of Bad Guy - and the ability to knock his pee pee in the dirt, it's probably a good rifle if you can get a good price on it.
That said, it's still a "factory" gun and for that kind of money, I would like to have more say into the components than you are going to be able to do since Sig isn't "standard" in what they do....
What's the guy asking for it if you don't mind me asking?