The Technokratos guide to buying HDMI cables.

This is the most simple guide you will ever read:

Buy the cheapest one possible.

See? That was simple. There is a very good reason for this, one that the Currys/Comet/PC World staff don't want you to know: there is no difference between cables. None at all, for the thing they are meant to do. Here's why:

HDMI is a digital standard, so there are only two possible signals that can come across each wire: 0 or 1, Low or High, ON or OFF. Simple as that, it doesn't matter whether the 1 signal is 0.1V off what it should be, it still counts as a 1. (The 0.1V difference can make a difference in analogue cabling, so for that the quality cable necessity has some, albeit very marginal, evidence). Even if the cable does cause a 1 to be corrupted as a 0, the HDMI specification means that there is error-correction so it will be corrected before it actually is made into video or audio. Also, by Monster Cable's logic (they make £110 HDMI cables and incentivise shop staff into making people pay for them), a 'poor quality' cable would produce unwatchable videos, and here's why:

HDMI can transmit xvYCC 16-bit video. If there was one bit error in this 16-bit signal, there are 16 different possibilities for the outcome. If the error bit was at the end, then there would be a decimal difference of 1 in the received signal. If the error bit was at the beginning, then it would be very different indeed: a decimal difference of 65536. Have you ever seen unwatchable video like this over any HDMI connection? I thought so too. To provide a 'test' to prove expensive cable's superiority over 'lesser' cables, they provide 'fair' demonstrations, like this:

Monster "HDMI Difference" scam still kickin' in Fry's Electronics - Engadget

So, for the TL;DR crowd: don't be fooled into HDMI cables that cost more than £10 at any costs; you can get £3 cables that work just as well. And probably better since they don't need to pay for marketing fluff.