I've always just rocked my one pair of casual sunglasses biking, but thought I'd give the sporty shades thing a try. At first they seem so light and flimsy that I didn't think they'd stay on my face. On the contrary, they're grippy where they need to be so they don't budge whatsoever even riding down rough terrain. And between the crazy light weight and the lack of frames in your field of vision you can barely tell you're wearing sunglasses.
Getting three lenses makes it a great value, although the photochromic ignitior has worked so great in various conditions so far that I don't see myself having to swap lenses too often. The process of swapping is super easy which is nice and they include a great case to safely house your spare lenses (and full glasses while traveling).
They're fairly expensive, but my justification was that I was getting better eye protection and relieving my similarly expensive casual sunglasses of the abuse of mountain biking. This "max" model is definitely fairly large, but I'd say anyone with a medium to large face could pull them off no sweat. If you consider yourself particularly small-faced though I would look to the regular pivlock v2s.