I don't know how others manage that, but with my 3 or so reviews (the reasers seem to be as lazy as myself ) I can only talk from little experience - and so far there was only one review I really knew something to say to it, although I once tried thanking the reviewer per PM. At least with feedback emails I try to send a "thank you" back, but often there is nothing more to add (someone already said it, what can you say to "please write on" or "I liked it"?)