To view a longer list of Amazon products click Ab Machines
Ab machines can make one's life much easier. It's good for everyone, especially fat people or people with a weak back. Yet, there are many on the market that are worthless or not as useful as others.
For example, the Ab Wheel will work your abs and shoulders. I would say the best of the many wheels is the Perfect Fitness Ab Carver Pro.
Yet, for another $25 dollars you can get the Gold's Gym AbFirm Pro. I had purchased this and it is good, but not enough tension so I had to return it. Therefore, if you don't have a strong stomach this would be good, but if you have a six pack or even just be able to feel your muscles this would be a waste of money.
Then you would want something like the Valor Fitness DE-4, but that's a bit pricy, $251.27. Yet, if you really want a six-pack and a strong back and you have the cash, then you would want something like the Nautilus Abdominal-Low Back Machine. If I could afford it that's what I'd get.
Aside from that, many ab tools, like the wheel, can put stress on parts of your body that should not be worked and in turn injuries can occur. If you want a six pack then you want to isolate the muscles of the abdomen and the wheel does not do that.
Don't misunderstand me, you do not need a six pack to have a strong back, but you need a strong stomach to have a strong back. The abs and the lower back are connected and the strength of them are can affect your ability to walk. A good tool that will strengthen your abs and back, much more your back, is the Stamina Body Trac Glider 1050 Rowing Machine.