Sunglasses are sunglasses. They cut the glare and protect your eyes. Besides that, they make you look cool. I would surmise that most cyclists choose their sunglasses based mostly upon design and fit. I have gone through dozens of over the years and I have not noticed a huge difference between the extremely pricey to entry level models. They all have similar plastic frames and polycarbonate lenses, all of which claim to be superior to the next.
Lack of visibility while in the drops or aerobars is my biggest issue with most models I have tried. I ride a fairly aggressive position, and most cycling-specific sunglass frames block my field of vision. My second criticism would be lenses fogging and/or getting covered with sweat in humid conditions. Slipping off the nose is another annoyance, and having to choose and change all those lenses for the varying light conditions is my final issue. Does anyone really do this?
Ironically it took a bicycle manufacturer to address these issues and produce a superior product- Specialized. I recently tried their ARC II model and they will be my sunglasses of choice until someone improves upon this already great design. For starters, the lenses adapt to the light conditions. This is a big plus for me as I often ride at dusk and of course don’t carry extra lenses for the changing light conditions. This feature in and of itself would have sold me. Additionally, the lenses did not fog even in misty conditions in which I often remove my glasses entirely. They are also extremely light weight and stay on my nose no matter how much I sweat. In fact, I hardly know they are there. The lenses are made of NXT, a material used in bulletproof applications on Apache helicopters and purportedly 10-20% lighter than polycarbonate. NXT is also an unbreakable material.
The lenses are also designed to optimize the pantoscopic angle specific to cycling. This means the field of vision is adjusted and angled for the unique head position while on a road or mountain bike (thus road and mountain models). For instance, the road versions position the lenses higher in the forehead so that you are not looking at the frame or over the top of it when in the drops or aerobars. The ARC II model covered nearly my entire field of vision, both forward and peripheral. Again, you forget you have them on.
Another unique design element is the rubber sweat bar that runs along the top of the lenses. It keeps sweat (and salt) from running down the forehead into your eyes and onto the lenses. It is an additional point of contact and stabilizing the pressure on the face and reducing it on the nosepiece.
The only possible recommendation would be to offer folding frames, as they currently are one-piece models. However, this made no difference to me. Each ARC II comes in either a small or regular model with Adaptalight (light changing) being an option, although I don’t know why you would not opt for it. And there are enough colors, shapes, and sizes to make you look cool, they are priced reasonably, and come with a hard case.
Specialized definitely did its home work on the ARC II.