Ron wrote an answer about Oakley M Frame Sweep Replacement Lenses on June 23, 2008
On a bright cloudless day, you can tell the difference. I got the slate iridiums and they work fine as a multi-purpose lens for cycling in both bright and overcast conditions in Southern California. You probably won't need the black iridiums unless you're out all day in the desert. Once it starts getting overcast, the black iridiums start hiding things in the shadows.