The ab crunch machine is good for anyone who is tired of doing dozens of crunches. To properly train a muscle, you need to apply enough pressure on it, which cannot happen if body weight exercises are too easy for you.
This is where the ab crunch machine comes on, it lets you adjust the weight you use, and apply the right pressure for your rep goal.
But there are some drawbacks, With this machine it is crucial to adjust the machine correctly for your height and body. Failing to do so, might cause your workout to be ineffective, and will increase your risk of injury.
Also, crunches of any type might cause lower back pain, So it is better to do a different ab exercise if you have trouble with your lower back.
Make sure to use weight you can handle, choosing too much weight can easily cause injury.