Just to throw an alternative opinion into the mix....
I have been a massive fan of Oakley over the years, M-Frames, Radars, Half-jackets, Minutes, Racing Jackets (plus several others) and at least 3 pairs of snowboarding goggles. The M-frames I also use as safety glasses, but what has annoyed me about them is the lenses delaminating after a few years, like a coating on the inner surface coming off. Considering you pay ~£50 for a replacement lens, I would expect them to last pretty well.
I have therefore started getting replicas from eBay for ~£25 for a frame + 5 lenses including polarised (Radar) which are indistinguishable from the real thing, and aren't an issue if you scratch them. At that price they can almost be treated as disposable.
I know the lenses won't be as good as the genuine article, but from a value POV I can't fault them.