Hot rollers give you a different kind of curl. Curling irons kinda give you a flat squished curl while the hot rollers are kinda looser. They take about the same time but without as much effort because you just put them on, get your makeup on and take them all off at once. They have to be on your hair for at least 15-20 minutes or longer depending on the length of your hair. If you don't leave them on long enough then your hair isnt going to curl as well and you may have flat spots.
My set only has a few large rollers, not enough for my whole head. I would do as many as I can, take some off, heat it in the unit and apply it in another part of my hair. Damp, not wet hair works best.