I know that this is going to fall on deaf ears (ignorant eyes?) but after replacing one of my cheap HDMI cables with a far more expensive AudioQuest HDMI cable, I can safely say that it has definitely made a worthwhile improvement to the sound. I am unable to comment on picture, as I would need to change the cable out of my Processor to my Projector to judge that.
It is obvious that many people don't believe that cables can make a difference, but most of these people either don't have the equipment to benefit from the potential difference that 'better' cables can make, or have been led to believe that a difference is not possible and have therefore closed their minds to the possibilities. The fact is that if your equipment is not capable of revealing the difference in picture or sound quality that a cable can make, then you obviously are not going to see or hear one.
A similar thing could be said, for example, about adding an expensive DAC between a bog standard CD and Amp. While the DAC may be very capable of allowing a more faithfull 're-construction' of the source material (compared to the DAC in the CD Player) if one or more of the other components in that system is dragging everything else down, then you are not likely to notice much or any benefit. If you have a bottle-neck, you have to address it. Otherwise you are not likely to realise the potential benefits other changes can make, which will lead you to believe that such changes are pointless.
It is true that not all cables will make a difference. Also, a more expensive cable is not always a better cable. But trying to claim that differences in cables is all a load of rubbish, just because you have not seen or heard a difference for yourself, does not mean anything to those of us that have seen or heard it for ourselves.
If cables really didn't make any difference then there would be undeniable proof and threads like this would not exist.
Source: Experience and common sense.