I had the chance to put a couple of rounds through one. The furniture felt flimsy to me. The stock felt like an after thought. The gun shot well but don't know if it is an improvement over the AR. I also heard rumors of problems with the charging handle but have no first hand experience with that.
Over all the rifle shot well but didn't feel very solid. I'm sure there will be a bunch of after market support but I wouldn't trade my AR for this.
I know this is four months late, but I have a SIG 556 and love it. Granted, only about 500 rounds through it right now.
As far as having loose fitting hardware (rails), I'm not sure I would agree with that assessment. It shoots a lot better than I'm capable of using. And, it looks great.
I've shot a few other ARs, but don't have enough experience to really compare. But, SIG does make quality firearms and that is why i purchased it.
My SIG 556 does feel a bit heavy to me as well. I'll be shooting it along with a couple of other ARs on Friday. It will be my first chance to really compare it with other ARs.
I have a buddy who used his in a carbine class last summer. It functioned without any trouble, but so did most of the ARs. It's too heavy. I didn't pay any attention to ergonomic issues, but you can't beat an AR in that department. If you have extra money to spend, it won't be wasted, but I'd rather have an AR.