The ab wheel can be very dangerous, and you can hurt yourself very easily. Espeically if you roll out too far, it will pull the muscles in your lower back. Also if you lose your balance for a moment, it can pull muscles in your lower back. And I don't mean to just tighten the muscle in the back but to injure them and put you in bed for a week or more.
The concept of the ab wheel is good, but if you do Cathe's Core max, you'll get to work the abs 10x harder, and you'll get results quicker, as core max actually work the full abs, upper, mid, lower, and oblques. Where with the ab wheel you mainly work the lower and obliques. You also got to have good arm strength and back strength for the ab wheel, for it to work the best you got to push youself out there where you holding all your upper body weight up, and then try to pull yourself back in with the abs. And that's another thing, it's really easy to engage the hips and glute muscles to help the abs so you don't get all isolated ab work either. As well as it's always pulling back. Where the best ab exercises are actually like the reverse crunch, or double leg stretch, where the abs are tightening the abs down against your spine while pulling your ups up off the matt and legs straight in the air. Or stretching out, and then pulling legs and arms into the chest area which works all 3 sections of your abs at once.
Hope that helps,
Kit