I have the Smith V-Ti
http://www.smithoptics.com/V-ti_30_234.html
I bought them primarily for mountain biking, as the lenses completely enclose my eye area. Great protection. The frame comes with three lenses: polarized platinum, Ignitor, and clear mirror. The clear lens is great in the woods, for eye protection without affecting visibility. The Ignitor is great on overcast, flat light days. The polarized platinum is more for snow and sand, with bright sun. Changing lenses is easy - I can keep a spare in my jersey pocket if I'm going to be riding in the woods and then out in a meadow, for instance.
On the down side, they're crazy expensive. And the Smith lettering at the bottom of the lens is sort of annoying at first, but you get used to it.