I've been buying them for ages and have had no problems with them (one fixed a work telephone, when I was working). I get full whack out of the router with them, so I'm not complaining. I do, however, avoid USB cables. I did manage to get some 10m Belkin gold plated cables for £1 in poundland a couple of years back which I haven't used yet.
Cheap, cost effective (IME), easy to get hold of, no faffing about with bits of wire and RJ's, reasonable length and money is tight for some people.
You can pay lots of money and still get low quality rubbish.