Fit is the most important thing. My cheapo ones are all far too big for my face/head. Cheaper ones usually trap my eyelashes. They also tend to rest against my forehead (making me sweaty) yet they do not come in underneath the eyes sufficiently to keep my eys from watering with the cold air streaming past. Cycling glasses should be a snug fit but not actually resting against your forehead, they should be comfortable on your nose (ones with an adjustable nose peice are useful if you cant get others to fit well) and they should be contoured to your face, coming in underneath and around the eye sockets, so that particles dont fly into your eyes and your eyes don't water with air flow. I suspect that more expensive brands actually do some research into the 'average' face shape and use computer software to base their designs on that. Of course if your face shape or size differs a great deal from the 'average', you will then find it harder to get glasses to fit well. The Oakleys I now wear have an anti-fog design (which does work) and they also fit round my face much better than any cheap glasses I have previously had (including safety glasses). Oakleys are expensive, but you do get great after sales service. If yours break after the warranty runs out, they will do a part exchange or offer replacement pairs (of newer models) at the same sort of price as the very cheapest cycling specs, once you have bought your first pair at full price.