I've had great luck with Oakleys and Ray-Bans, but I typically purchase mine online or at Nordstrom Rack for a substantial discount. I don't think they're worth MSRP.

That being said, their coatings don't come off; their coatings are consistently applied; their lenses are typically distortion-free (so they don't give me headaches); and it seems to me that their lenses do a little better at resisting scratches (although that's very subjective—and maybe I treat more expensive sunglasses better/protect them from scratches better).

The anti-distortion thing is the main thing, IMO. It's not easy to bend a piece of glass/polycarbonate without causing distortion. But if that's something that you don't notice and doesn't bother you, then by all means get the Pit Viper knock-offs and forget about this whole thread.

If you care about such things, more expensive sunglasses also tend to have interchangeable lenses—useful in certain situations (like when I'm mountain biking and want eye protection, but also don't need full-on sun protection in the last 90 min or so of daylight).