The degree, if any, to which the advertised card "speed" makes any difference depends on the camera.
I don't think it makes the slightest difference in any of the standard sub-£300 miniature cameras. It will probably be noticeable on a top end DSLR and could be relevant if you are taking pictures in a rapid sequence.
I have had a number of cameras, up to a £1000 DSLR, and have never noticed anything obvious. I always buy the cheap cards.
Just once, I bought a "special" 256MB SD card which was about 4x the normal price, which supported some sort of nibble mode; I never looked up the specs (too lazy, despite being an electronics design engineer) but it's basically a higher speed mode which enables the data to be loaded in bigger blocks at a time. Most of the flash memory chips used in these cartridges support some such mode. I found that the only device I had which took advantage of this high speed mode was a particular PDA, whose backup function ran about 2x faster - hardly worth the extra cost.