I won't buy Sigma for just that reason. Way too many people say you need to keep swapping out until you get a sharp copy. Screw that. I've read the same about most Tammies, but the 24-70 seems to have a pretty solid backing.
I can personally vouch for the Rokinons. I own 3 (8mm f2.8 FE, 35 f1.4, and 85 f1.4) and they're all sharp as a tack. The 85 f1.4 is the weakest of the bunch, but only as far as metering. It's still quite sharp if you get the focus right.
"I know rocks. Rocks are hard."