Straight after the conference, I enabled validation on a resolver that handles in excess of 2 million queries/day.
So far I have no negative impact to report either :)
I support Andy & Dean's comments. Just turn it on already!
Our experience matches too, our two recursive resolvers combined have a similar load to that of Pete's.
We've been running recursive resolvers in excess of 6 months now. �Our audience is mostly hosted servers and VMs who don't tend to complain about inability to view cat pictures, we have not had problems with resolver service.