Cables can make subtle differences, but they are often seriously overpriced, and more dramatic changes can be made by changing componants. But if you have a system you really like, but its not quite perfect, a cable change can make the differnce.
And does it even matter if it is a placebo effect. If spending a few quid on some cables gives someone satisfaction, and they end up enjoying there system more, then who's to say it was bad value for money.
When I put my system together, I tried various CD players, amps and speakers, and once I got a combo I was happy with I took it home, alone with a bunch of cables from the shop, and just tried various cables. I spent 2 weeks with a box of sample cables, and selected the onces I liked the best, based on both listening to the system, and also the construction quality.
They were probably a little overpriced, but that didnt bother me back then, and now, 15 years later, not one cable is showing any sign of ageing. The gold bananas, and RCA connectors are still firmly attached, and being soldered on there is no problems with oxidation (and no dry joints)
While I wouldnt say there were any "spectacular" cables, some cables definalty caused my system to sound bright and harsh (Hey its an Audiolab

), and other cables caused my Mission Freedoms to become loose flabby bass monsters. But there were cables in the middle which seemed to just let the system sing, not to bright, and able to bring the bass under control.
As I handnt handed over any cash for cables at that point, my selection wasnt really coloured by the fact I had handed over my money
Funny thing I do have to mention though, I was in need of a single long cable for a sub, and I was in a hurry, so picked up a "Monster" cable from a highstreet shop. Worst purchase I ever made, the RCA connector is extremely cheap, and it makes a very poor connection. High price is not a good indication of a quality product.