I am still new to the weightless senko's and line watching...it is a work in progress for me, as I have always been taught to keep the line fairly tight to feel the bite. I feel I am getting better at leaving it loose and watching the line, but I still have problems with reeling up the slack and automatically setting the hook when I see something. Having read enough posts to see that it's the correct thing to do, it is merely a confidence issue with me I guess. It's always been so drilled into my head to tighten up the line and wait for the fish to run, that I'm always scared to swing and miss. As I said I'm working on it.
Back to the line issue, I haven't found it that difficult so far to see the strike on my line...that's actually been a pretty easy transition. To me, wherever I am viewing the line at on top of the water, I can see the strike...it seems to me that when they hit it, the line reacts at all points in its length, but, echoing what another poster said, I have found it more difficult when the fish picks it up and runs toward the boat with it, especially if it's not completely calm on the water and the boat could be floating in.