I ride about 200 miles a week and have gone through several pairs of glasses in the last few years.  At times if they get uncomfortable, or if the sun goes behind a cloud, I'll slip them onto the collar of my jersey to give the eyes some breathing room for a few minutes.  A bump or two later and they're chattering away into multiple lanes of traffic (I'm pretty sure glasses are designed to bounce in the direction of the most possible traffic in any given situation).

Anyway, tired of looking for deals and trying to keep things affordable, I gave these a shot.  I was certainly hesitant to purchase online for obvious fit reasons.  That being said, I have a larger head (I wear a 7 3/4-5/8 ball cap), and these fit perfectly.  They feel loose... but don't budge once they're on.  They don't slide down my nose, and while they're dark enough to sheild from the sun, they're certainly light enough that I have had no issues riding in thunderstorms when the sky is darkened.  I wouldn't wear them at night... but they're not magic, so I don't expect that.  They are also extremely comfortable, so between that and the nice level of shade, I don't think I'll be taking them off nearly as frequently.

For $20, I'll certainly be buying another pair when I break these (by my own fault) in about 6 months.