"Firstly, this rifle is fantastic. It is as close to the real thing as you can get. Having experience with the real rifle, as well as the Tokyo Marui Sig 552, the JG SG 550, and the ICS Sig 551, I can tell you that G&G is the far superior Sig replica.
The good: The rifle is is steel where it should be, and polymer where it should be. The stock is solid, with no wobble. The rear sight drum has smooth, positive rotation and lock. There are even glow in the dark inserts in the rear drum and front night sight to simulate the tritium sights on the real rifle. The take-down pin not only serves to remove the front hand guard and access the battery, but the rifle also pulls in half at the lower and upper receiver to access the gearbox, barrel, and other internals. Firing the gun feels great. The gearbox isn't whiny, and it just feels tight.
The bad: The rifle came with a solid orange, plastic SCAR flash hider, pinned and glued to the muzzle. It did not come with the as advertised Sig 550 metal flash hider. The front take down pin (as mentioned above) was too stiff to remove by hand. It required a punch and a hammer to remove and replace. The front night sight's detent is also super stiff. You will hurt your finger tip trying to flip it up and down. Inside the front gas block, there is a large black locating tapered detent, that locks the false gas tube (battery compartment) into the rifle. This is also very stiff and gets jammed up easily.
Other comments: Despite being 'licensed' by Swiss Arms, this rifle has absolutely no Sig marking on it anywhere. The rear sight is actually welded to the receiver (like the real weapon) which means that any rail mods which require the removal of the rear sight will require a permanent modification to the gun (you'll have to cut off the rear sight). The dove tail block on the top, front portion of the upper receiver does not fit my Tokyo Marui Sig 550 rail. I got the 7.4 LiPo with this gun, and it's a bit weak. It also doesn't fit quite right in the false gas tube, being a rectangular battery sliding into a cylinder. You'll have to force it.
All the bad things mentioned I believe can be 'broken in' or made to work, but I think should've been reliably functional out of the box. Honestly, the flash hider was my biggest disappointment. All of the good things make this rifle worth getting though, so it gets a 4/5.