Provided they are free of any actual physical defect they'll be fine. There is no scientific evidence that I'm aware of that shows that a £100 HDMI cable trasmits a digital signal even remotely differently than a £20 cable! On the other hand plenty of people will swear blind that their £100 cable produces a picture that "looks better" to them, but then so would I if I'd blown a hundred quid on a digital cable!