We've been wearing ESS eye pro (http://esseyepro.com) since the beginning of our career. The first thing we look for in shooting glasses is safety, and there is no tougher glass than ESS. Their ballistic glasses can be shot with a shotgun from 10 meters aways and they won't even shatter. Now hopefully you'll never be shot at, but that gives you an idea just how well made these glasses are.
Another factor we look at is lens options. We change our lenses based on the conditions of the day. Whether it's sunny, cloudy, overcast, or whatever it is, ESS has a lens to fit the situation. The ESS Crossbow offers many lens options and you can change the lenses in a matter of a few seconds.
If you care anything about precision, you should be looking for a lens that has the best clarity possible. Lenses bend light and can distort where things appear to be. There can also be distortion in lenses. Both of these conditions can lead to eye fatigue. ESS has very good quality and that is why we can wear them as we shoot all day long.
Speaking of all day long, if you're going to wear classes all day long, they need to be comfortable. ESS glasses are made so that you can wear them all day just like many of our military do.