I have 25K Macs and 13K iPads with query_cache_size set to 16M. This is a recent setting so I have only had it under load for about a week now. I set query_cache_type to 1 and size to 16M just before Christmas break mid December. I have not seen any negative impact but can't really say if it has helped any yet either. I may let it ride like this for another week and then bump it up again to see if it makes a difference.
@Josman you have probably already seen the MySql reference manual. However, for others that have not there is some great info in there. Also, at the bottom is a link to some additional info on query cache settings and what to monitor to base your scaling on.
