I have the 70mm/2.8 EX DG (right now, Sigma offers a newer model). It is really very sharp up into the corners (stopped down to F/5.6 or higher), no difference between center and corners. Did not try it wide open, also because the DOF is too narrow.
By the way: this lens is built for FX format, so with the DC format D90, you use only the center part of the image circle!
At the time, it was tested in a German photo magazine, ColorPhoto, and came out as the best in its class, both on Nikon (D300) and Canon (EOS450D). On Nikon, AF works only on cameras with AF motor built-in. Not on the D5300, for example.