I leave kudos on about every story I read but leave comments on the ones I really liked or loved. I think kudos is a nice way for a writer to get a ballpark figure on how many people have read/finished their fic, as the hit count can sometimes be misleading. Most of the time I comment too, but commenting takes me much longer (usually several days to think it over) so I like to leave kudos as a kind of "I liked it, comment to come!" promise.