The fly still holds a permanent spot, actually in all my fishing. I use it in the salt, where it seems to do best in September and October. I also use in for trout in freshwater, still and running. Tied as pr. the orginal (OK, I use synthetic tinsel, but otherwise...). It does well in the rivers for sea trout and salmon - also late in the season, where I tie a modern version on tubes (I think it's somewhere in the archive). And finally, I also tie a huge, modern derivate for pike, where it lights up in murky waters.
I know there are variations with hackles etc., but I've always loved the simplicity of the original. I've also tried substutuing the bucktail with other types of hair, but for the hook-versions, nothing beats good, old bucktail (which by the way is experiencing a well deserved return to modern fly tying).
The fly has a long history and is surrounded by a lot of myths, and it's inception is lost in the fog of time, but it goes back at least to the 1930ies, where it was popularised by John Alden Knight - first named The Assassin and later on renamed the Mickey Finn. You can read the story about it in Jospeh D. Bates, Jr.´s book, "Streamers and bucktails" from 1979.
Have a great weekend and tie a few Mickey Finns!