No limitations on distance. Everyone seems to think that it's strictly a short-line thing. The only time it won't work very
well is if you are fishing straight up or straight down stream....and actually you can still do it, it's just requires more skill (which I don't have BTW, so don't get me wrong, I try to avoid straight up or straight down if I can).
Once the drift is set up right anything that stops the fly (even for a mili-second) causes things to immediately tighten up. I can definately detect THAT happening at a distance way easier than I can see a bobber twitch at lets say 35-40+ feet away. The hook is usually already in flesh so all you have to do is finish the job. Your focus remains on the angle of the line between the rod tip and the water surface....instead of the little orange ball.