I haven't tried one out yet, but I have been reading a few reviews. Cost is kind of high, but Ruger really put some thought into SR 556 and did a great job of outsourcing some quality add-ons. I would be really interested as well with some first-hand experience. If it turns out to be a winner, I would opt for the Ruger SR-556 over the Sig 556...solely due to my familiarity of the M4 platform.
I don't know anything about it yet, but if it lives up to Ruger's reputation, it should be one reliable and durable machine. I've always said that Ruger has made mediocrity into a fine art, but I give them their dues when it comes to reliable and durable guns. So far it's a bit overpriced, but I guess only time will tell as the newness wears off and street prices adjust to the market.