I believe that there is quite obvious differences between some types of cables, but not others.
I always thought my system sounded dull about 6 months ago. So I changed my speaker cable. Ever since, it has NEVER sounded too dull. Go figure, a quite obvious difference, and one that is real since I have been happy ever since. So, speaker cables are VERY important, but not neccesarily always the most expensive...
Interconnects are more subtle in their differences. The only interconnect that has ever made anything more than a subtle difference, is the solid silver one I am using now. Some of my others seem slightly better than others, buts its very subtle.
Digital cables, well, I've had two, and if there was a difference, it was minute, and I definetly could not ABX the difference. Placebo?
Mains cables, it depends on the cable, and the component. On my old DAC, I had a Signals Blackcurrent and that made a very noticable difference to the bass. A Eupen cable on the same DAC, made a smaller difference. Neither of the cables seem to do anything for my amplifier. However, on the Corda HA-1, the Eupen made a big difference. So, thats real, but I think it depends on the quality of the PSU on the equipment. Poor PSU's should benefit more, better ones should "reject the crap".
I do also believe its at least part placebo, and that the differences quite often arent as big as we are sometimes led to believe, but they definetly make a difference in many cases.