My experience? Aspherical surface usually means a sharper, more contrasty image wide-open, especially with a high-speed lens. Comparing an ancient Canon FD 55/1.2mm with a FD 55/1.2 aspherical wide-open, the difference is dramatic, with the aspherical being much better. Of course many will argue that the signature of the non-aspheric is better. But that’s a personal choice.