For or against? If "for", what do you use? I've been using crappy ones for a long time, and I noticed one really good reason to step up to decent ones - the crappy ones affect autofocus in dim light. Not drastically, but when it really counts the reduced transmission (who knows how much...1/3 of a stop or more?) can make even my better lenses hunt. Did some research, and ordered a couple Hoya HMC UV(0) off eBay. They are supposed to be tops for visible light transmission, UV reduction, and are decently coated.