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.