I am fairly certain you are talking about the wheel roller type ab exerciser. I do not recommend them.
The vast majority of users do not have the motor control to use them properly. The amount of stress that the movement places on the low back is not reasonable. Even someone with very strong stabilizer muscles and abs can relax for an instant and seriously injury their low back. And the relationship of the position, movement, and manner in which the spinal stabilizers are engaged and working to a movement that occurs in daily living is virtually non-existent. I see using this exercise device in the same light as standing on a fit ball. Why? What are you really training that can’t be trained more safely and more effectively?
I recommend that anyone considering such equipment or any exercise that they are inexperienced with, take the time to evaluate the exercise biomechanically. If you are still interested in the usefulness of the exercise after determining the potential drawbacks, design a system of progressions from the most basic static positions of the exercise to the full movement. Then you perform the progressions yourself and re-evaluate your design. When you are certain that your progressions are safe and effective, you can begin teaching this exercise to clients, The re-evaluation process continues as you learn how clients respond to the progressions and if there are any reasons for clients not to perform the exercise.