If you're totally out of shape and you plan on buying an ab machine, first you should know that ab machines will work only if you are already doing it along with some kind of other work out like cardio, weight training or if you do sports for example. These machines aren't enough by themselves and they won't get your whole body ripped like the models that come out on t.v. These machines are designed to work out just your abdominal section and won't make you burn enough fat to get that lean and sexy body you want. So before you buy one of these ab machines, make sure you are following a well balanced healthy diet and at least you're cardio training.
If you're already skinny,lean and have a low percentage of body fat, ab machines will work for you because it's easier for your abdominal muscles to show but these ab machines by themselves will NOT make you buff/ muscular. In order to get buff you need to do weight training. And the disadvantage about these ab machines is that there's repetition to a certain movement so you'll only be working part of your abdominal area. I say it's much better to different kind of ab exercises without using any machine. They're even more effective and you save time and money instead of doing the same boring workout on any ab machine.