I love flyes. I do them every other chest day. I know soreness is not supposed to be a sign that you had a good workout, but I can bench heavy till the cows come home and not even feel it the next day. The next two days after having a 'fly' workout, however, my chest is killin' me. Could be because it is more of a stretching than a pushing motion, I suppose....
Mental, maybe, but it's always good to have variety in your workouts anyhow.