I've used bead chain eyes on my shad flies for a long, long time now, but along the way, came to use them on some of my streamers for bass and my little bugs for bream. They've always seemed to add a little something to many of my ties. They help sink a fly, of course, but those bulging eyes have a simple "magic" about them that I think may help as well. It's always conjecture to talk about what fish think, but what they do is observable, and it's always seemed that most wildlife, aquatic or otherwise, often seem to kill the genetic anomalies that sometimes appear. Those big, bulging eyes could be seen as a genetic mutation that needs to be eliminated, and with fish, having no arms or legs, he only way to eliminate them is to bite (and usually swallow) them. That's my theory, anyway, and I guess I'll stick with it 'till something better comes to mind.
With nymphs, it seems to enhance their performance as well. I also like them on small crayfish/scud type flies for small species. I usually weight them at the rear, like a shrimp fly, but sometimes in front. They tend to work well both ways. I've wondered if a fish's brain is too small to notice the difference, at least at times. The placement of any weight, like bead chain eyes, affects the fly's balance, and therefore its action in the water, and that's what I use to guide my instincts in creating flies with BC eyes - always asking "what do I want the action and fall to look like?"
Do any of you do something similar?