I think most ab machines are a complete waste of your time. How dare I make such a claim when there are hundreds of ab products out there?
All I can say is the fitness companies are giving the public what it wants. Everyone is screaming for flat abs, so of course there are going to be products sold to meet that demand.
Here's the truth: Most of us have nice abs under a layer of fat. You use your abs all day long as a stabilizer and one of the best ways to train your abs to be flat is to think about holding them in throughout your day. But until we lose that fat on our abdomen with a sensible diet, challenging cardio, and effective weight training to increase metabolism, a million ab curls won't make our abs flat.
Am I saying you should give up all ab exercises? No. I'm just saying to focus on losing the extra fat and train your core with effective exercises like plank, side plank, bicycle, captain's chair (lifting knees while suspended in the captain's chair), and crunches on a ball (amazing that adding the ball makes crunches effective, but it does because of the instability of the ball). You don't need to spend a lot of money or a lot of time on your abs. Five minutes a few days a week will be enough to give you good core strength and make those abs beautiful when those last few pounds come off.