Sometimes they work OK and sometimes not - luck of the draw... As long as you are aware that 18" is the IEEE spec for UDMA/UATA cables and that anything beyond that is at your own risk, go for it. Most of the cables out there these days are made so cheaply it's a wonder they work at all and you can't even be sure of better performance if you pay for a premium brand like Antec Cobra, Monster or Granite. But at least with the premium brands you should be able to get your money back if they don't work for you.
.bh.