There are lots of people who strongly contests that expensive HDMI cable has improved their Audio/Video quality. I'd confidently say... it's all in the mind....
I have never been a believer of snake oil or those expensive cables. Actually I have been continuously comparing expensive cables with one that I think have been made with reasonable care and attention. But I do strongly believe that a cable made with care and attention by a well know company has some value.
I have two examples. In my son's room I have an Akai Flat TV connected to a Pioneer DVD player. For a long time I have been using an ordinary component cable that came with the system. Some time ago, I went out and purchased a Bandridge cable that made a marked improvement in quality, though I continued to get noise and flicker. Recently I picked up a reasonably priced British component cable from another member. I replaced the cable and the difference was palpable. My wife actually asked me if I had changed the DVD Player. The blacks are completely black, and the noise and flicker have completely vanished.
In my HT room I again had ordinary HDMI cables that came with the players such as Oppo or Pioneer. Recently, in an experiment, I changed the cable between my HTPC and the AVR to a HDMI cable from Audioquest. Again the change was palpable. No flicker, no noise, absolute blacks and I can clearly hear every cricket chirping in an movie. The low frequencies sound absolutely delightful with a sharpness and speed that the previous cables could not deliver. I am slowly replacing all my HDMI with AQ. Mind you, these are entry level from AQ.
HDMI cables, in addition to carrying digital data, also have transmission speed to take care of. In addition, the way the connectors are made are very critical for your equipment. If you have a badly made connector it could damage you HDMI port in a player or receiver and that cannot be repaired.
Choose your cables carefully. I would avoid unknown brands and really cheap ones whether you are using it for analogue or digital transmission.
Cheers