The 556's have had their share of quality probs if you get a good one you will like it.
I love my Sig 556. I have THE Original model; the first one to roll out of production. I waited just long enough to let them clean up their QC problems and then I pulled the trigger. I'm very happy with mine. It'll eat anything I feed it. To be honest, I think it is the only gun in my collection that has never had any form of malfunction and I've run several thousand rounds through it.