For around 5 months (up until last month) I (well, my Dad and I) used a very highly thought of Component switch box (a proper one, not the one linked in this thread) to a Sony KDEP42XS1 Plasma screen and Sky HD, via 2x QED Component Cables and 2x Black Rhodium optical cables, also included in the set up was a 3rd QED Component Cable to our DVD Player and a Nintendo Wii via Official Component (sorry if that all sounds complicated).
We were pretty happy with the HD picture, although I felt it had lost a good bit of clarity, sharpness and contrast immediately, it wasn't until we unplugged it all and used 1x QED Component cable directly from Sky to the TV that the obviousness of a clearly degraded image came through, it was total night and day.
Now the problem is, a lot of money were spent on those cables and switch box, they weren't your run of the mill switcher and cables, and if the picture worse on that, it's only going to be even worse on a lower quality cable and switch box.
Now you might not be too bothered, we weren't at first, as I said, we used it for around 5 months, it was only until we went back that it was how obvious the picture change was. However, if you're only using SD content via the switch box via Component, there is a very good chance you won't notice it, as it would be much more noticable through a HD source such as PS3, Xbox360 or Sky-HD.