"This is a sweet rifle and performs extremely well in a CQB environment. It outclasses similar M4 rifles in several ways:
1, The stock. I have found that while wearing full face protection (as is recommended/required at most CQB fields) the stock of an M4 gets it the way, making it difficult to quickly and comfortably aim down the sights. The stock of the SIG is angled in such a way to eliminate this problem with most of the masks I've worn. If the mask still hampers aiming, the stock can completely fold to the side, restoring accuracy. M4s obviously can't do that.
2, The sights. Most airsoft iron sight that I have encountered have been small and and difficult to use, prompting me to buy an optic for most of my rifles as soon as possible. Not so with the SIG, its iron sights are large, open, and well defined, making aiming easier and making optics more for looks than necessity. This could very well save you money on having to buy optics for it.
3, The weight. The SIG's externals are mainly polymer, which could be a turnoff for some, but I have found that this reduces weight, making it extremely light, fast, and maneuverable. Along with a very short overall length, you've got a great high-speed, low-drag rifle. The low weight also makes it a good choice for younger players.
While these are the main benefits in my mind, it does have some other good qualities. It's unique and will draw attention from the masses of M4 wielding players. It's ICS, a brand that has a reputation for quality, and is one of my personal favourite brands. It's completely ambidextrous, with both the fire selector and mag release accessible from both sides (sling mounts are right side only, but who really needs a sling in CQB?).
It is quite accurate out of the box, with a little more shot variation at longer ranges than the M4s I've used (note: this is comparing it to both CQB setups and more powerful field setups), but at CQB ranges it's accurate enough to hit the enemy every time. I would recommend only using .20s, with .25s there is significant bullet drop, and you'll find yourself trying to lob the BBs at the target. It shoots amazingly with .20s though.
There are a couple of downsides, however:
1, The battery compartment. It's really small. It fits my 7.4v butterfly LiPo fine, but anything larger (either wider or longer) and you will probably need a PEQ box or similar.
2, The mags. First off, the mag functions just fine, no problem there. But they are really expensive ($30 a pop), so if you want to run midcaps or replace the stock mag, it could run ya a bit.
3, The flash hider. I hate orange tips on my rifles so first thing I did was try to take it off. It does not come off easily and I ended up sawing it off. I'd say either just leave it or paint it.
A couple of other things to note:
The trigger guard is kinda weird. It can flip side to side, which I guess is good if you're playing in mittens. It's just a little strange.
The mags are of the "rock n' lock" variety, like an AK (though not interchangeable with AK mags). To make reloading easier, if you go for a foregrip, get a wedge grip. it won't get in the way as much.
Overall, I love this rifle. It's a great improvement over my previous two CQB M4s. I would highly recommend it to any player, beginner or experienced.