This is one of the machines that will waste the most amount of your time. With nutrition, genetics and body fat aside, ab benches (specifically the ones with arms rests) will do absolutely nothing for you. Why is that? Your upper body does more work than your abs.
This is really short and simple. If it has a place for your arms, regardless if it's a weightless bench or a plate loaded machine, don't use it. Go for ab exercises on the floor, a ball or even adjustable decline benches. All of these options will enable your abs to do the work rather than your neck, shoulders and arms.