Those of you that are experienced Canon lens users...of what benefit is the USM? I understand the autofocus on these lenses are supposed to be very quiet...but if I am not taking photos of weddings or other sacred/quiet events, is it worth the extra expense? OR is there something else that I'm missing? For instance, a 50mm f/1.4 USM is $520, while the 50mm f/1.8 II is $150. Is there something else that makes for such a difference in price? I know normally the faster the glass, the more expensive, but 1.8 vs. 1.4 doesn't seem much. Just looking to get the most for my money... Thanks for your help!