far better than flyes.
flyes only work your pec for about 1/3 of the motion. once the bell goes past the point level with your shoulders, it is moving more inwards than up. dumb bells only work when they are moving against gravity.
pec dec has force all through the motion cos its attached to a stack that moves up and down. if you arent feeling it, then you arent doing it right.
its the most isolated way of training the pecs, even better than crossovers because they dont have a cable that follows an arc. pec dec is easy on the shoulders also if you dont go too far back, and you can vary the angle by changing the seat height. you can also do partials from top down, or bottom up etc.