Sketchy, man, sketchy...
First off, good responses all round. Main thing I'd say is forget grey or greenish lenses for flats fishing. Period. I've proved time and again that they simply don't work as well as other lens colors. Clients that have the grey lense try their buddy's amber or copper lenses and can't believe the difference.
Personally, I've never owned a pair of real fishing glass, like Costas, Smith, or Ocean Waves. Certainly no Maui Jims (who can afford them?). I fished a pair of "vermillion" tinted Oakley 5's for a few years (until a small, leaky vial of 99% deet ate them alive). Most recently I use a pair of surfers sunglasses by the name of Arnette I picked up in Hawaii. The lens color is probably best decribed as copper or bronze and is a little browner than you'd expect. I fish lots of turtle grass flats and find they work great for highlighting fish. The design is wrap-around and they're unbelievably lightweight. Of course, all the drawbacks of polycarbonate lenses apply: easily scratched, deteriorate/fade in sunlight, etc.
Now, let me say, I've tried on several nice sunglasses over the years, including a pair of pinkish lensed Smiths that were
sooo amazingly clear. The difference in clarity that glass lenses offer is certainly worth it.
You'll always get spray on your glasses & when the saltwater dries, you'll have microscopic salt crystals on the lenses & invariably you will clean them with a cloth or tissue while fishing and thus putting micro scratches on the lens. Eventually, your glasses will appear "hazy" and your visual acuity will suffer.
I make it a habit never to arbitrarily wipe my shades, for any reason, without first washing them off. I completely agree about the salt spray/crystals. However, washing the lenses first usually takes care of that. I've had mine now for over 2 years and fish hard. (One leg is actually broke about halfway up but a little work with a candle, a metal pin - cut from a stainless hook - and some crazy glue fixed that.) Even with daily use they only have a few tiny scratches, none of which are visable when you put the glasses on.
So, here are some tips for cleaning glasses on the water. First, get one of those micro-fiber clothes. (Wal-Mart for a couple bucks). Second, buy some of that spray cleaner stuff. If you don't have any dump some water over your lenses or dip them in the water settling in the cooler.
If worse comes to worse - you're far from the skiff, just drank the last swallow of water and bones are still tailing, dammit, but that last fish splashed your glasses and now you can't see - lick your lenses. Yes, I said
lick. That will remove the salt and wet them enough to wipe clean. If you're good enough (I learned this from an old fisherman here) you can lick 'em clean and hold the lenses into the wind and they'll actually dry almost perfectly clear. Nice trick in a pinch... say you're wading way the hell down the beach with nothing but a flyrod, pair of shorts, and a hat with a few flies stuck in the sides.
Lastly, whether you use plastic or glass, always wash and clean your glasses at the end of every day. Salt eats almost everything, so the less time you let it sit on your glasses, the better.