Some call it the "weenie", others the "weenershnitlz", and the more erudite (or too ashamed) refer to is as the GW Emerger. For me, I always bless it with its full stream-side name - the Greenie Weenie. And I fish it without care or any possible shame, scold, or scowl. It catches fish. I guess some may think it too close to bait fishing, but I see it as simply matching the hatch. I'll fish it with graphite, with glass, and even with bamboo. I'll fish it on my little home limestone, out on big western waters, and if I ever get the chance, I'll even fish it at some fancy New Zealand or Patagonia lodge. I'm like that dude walking down the street proudly sporting a mullet and jean jacket rocking out to Bon Jovi on his walkmen while everyone else has tight pants, tight smiles, and tight ear buds listening to some highfalutin podcast about certified humane vegan smoothies. He's his own man, and I'm my own angler. I'll strut up to any stream, tie on a greenie weenie, and start slaying them trout (respectfully and reverently, of course).
I almost always fish it as my top fly in a tandem nymphing rig. I like to think of it as an attractor, where I'll drop a prince or pheasant tail or one of the other few nymphs I fish behind it on 12-16" of tippet. I think a lot of times trout will move to investigate the GW and then see the small nymph and end up taking that (note: these theories probably originate from my buddy Scot - see above about learning from the best). I've also landed most of my largest browns when they fly out of some little crease created by rocks on rocks and fast moving water to maul the hell out of it right as it hits the water thinking it's an inchworm dropping off a branch. Match the hatch!
I also like it because at the end of my drift I can let it swing like a wet fly, making it look like some sort of caddis emerging or something (I'm barely a competent fly fishing, so take my etymology with a grain of salt). Trout will nail it then and sometimes I'll even start to lift my line off the water for another cast only to realize there's a trout there (I always try to make it look like I knew the entire time....).
What it comes down to is this - if I want to consistently catch trout when nymphing, I'll use a Greenie Weenie. It's versatile, it catches a ton of trout in all types of water, and it's effective. Maybe, someday, when I actually know what I'm doing, the weenie will quietly take a back seat to some other fly, but for now, I'll keep throwing it out there with the same unabashed enthusiasm as a kid flying down a hill for the first time on a bike without training wheels screaming at the top of his lungs.
Here's a really great, soulful medley.