I just got my order from dummy ammo today and I wasn't overly impressed. They cycled ok but they are flared slightly at the front so they don't chamber smoothly. They fill the front with a thin coating of silicone and after cycling them a few times it is starting to let go a bit. It appears they also fill the primer pocket with silicone and then crudely wipe it off leaving silicone smeared all over the brass (I even had to peel silicone off the sides of the brass and plastic). All in all, I was disappointed with the quality. With shipping and taxes it cost me $50 for 20 and if I was to do it again I would just take some spent shells like you were thinking of doing and make my own.
If you have already ordered them, don't pay any attention to what I just typed

but if you haven't placed your order, well I just thought I would give you one user's opinion.