Unfortunately, I haven't had a chance to fire it yet due to the to crappy weather and bitter cold (it's -5 degrees right now in S. Wis.)
I have a friend of mine who was with the NATO forces in Somalia during the 90's and who has shot and extensively used the original Swiss Arms 55x series rifles. He handle this one and was impressed, saying that they improved on a few features of the originals.
I did research for about a year, debating the 556 and the various brands of AR-15's. I decided on the Sig after having fired a full-auto Colt with one of the local SWAT teams and having it jam repeatedly. I wanted to got with the piston system, rather than the gas impingement system. It was more expensive, but I think it will be worth it in the long run for reliability and ease of use during training.
If people here are interested, I'll take a pic of my collection of movie guns and post it. In addition to the three Sigs, I've got a Walther P99 Titantium (wife's), S&W M29, Beretta 92fs, Colt 1991A1, and an American made MP5 clone.