I turned it off the other day because I was fed up listening to frequent heavy disk activity (paying extra for very quiet fans in a PC doesn't do much good if the hard disk is frequently trashing around loudly instead).
http://www.vistax64.com/general-dis...-svchost-reading-large-files-whats-going.html
So far I haven't noticed any performance differences in any applications or games between having SuperFetch enabled or disabled (I'm not saying they might not be there, but if so we're probably talking split seconds on my system - or for very resource heavy tasks like games, perhaps a few seconds extra on initial launch), but I got rid of the noise

.
If you don't have that problem you should probably just leave SuperFetch enabled.