White flies aren't hexes. They are Ephoron Leukon.
http://www.troutnut.com/hatch/820/Mayfly-Ephoron-White-Flies
But to answer the question. YES. Although at the time I called them green drakes, only to later figure out they were indeed hexes. I've done well on the Allegheny and Kiski for bass/walleye/sauger/white bass/catfish. (though farther upstream, closer to Kinzua, they chase trout on the hatch). Note that these were with a spinning rod and a casting bubble. Walleye on dry flies is an interesting twist! We also used big nymph imitations on a spinning rod, though, during the day.
I've also had significant luck (fly fishing) for trout on some of the Allegheny tributaries, with the understanding that they were hexes, though I used GD dun patterns.
(For a while I was confused on why only central PA green drakes were white bodied as spinners and hatched around Memorial Day, when everywhere else they were various shades of brown and hatched from late June through late July. Everyone called them green drakes. But as I became a fly fisherman and informed that there are, umm, different types of mayflies, I realized they were a different bug. My "informants" in that area of the state still call them GD's)