I think they are friggin awesome. VERY smooth . One of the the smoothest 5.56 rifles ive shot to date. Its on my wish list. A bit heavy but it makes up for it with quick and smooth follow up shots.
I have the swat 556 and freakin' love it. I did end up swapping the handguard for the Halo style ( aka fish gill ) but this was just my preference. Only bit of advice, and this sounds odd, but sometimes the Sig magazines suck. Magpul Pmags and steel mags seem to run a little better in mine.
It's basically a 5.56 AK, with some minor changes to be more "western" in operation.
I know this site is a big trading community, beware that resale values for Sig 556s aren't the greatest. When I was behind a counter we had to constantly blow out used Sig 556s.