Hmmm, I can hear the difference, say between a standard optical and a cambridge audio analogue set of interconnects, but I'm not convinced that you get that much extra that spending the extra £100 or two on expensive cables wouldn't be better spent on a decent amp/cd player/dvd player, or whatever.
As for speaker cables, I can tell the difference between bell wire and 99p/m stuff, but not beyond that. Haven't tried biwiring speakers so I can't comment on that, but people will try and tell you it makes a difference.