Used an Ab Wheel for years, usually after a regular workout. Also good for tight shoulders, which most BB'ers don't realize that they have. I think of doing Ab Wheels as a reverse straight arm pullover, which also hits the abs and lats as the Ab Wheel does. And very good for a tight shoulder girdle.
Some men use the Ab Wheel starting from a standing position for higher reps (better men than I). Can buy a $5 Ab Wheel at WalMart or a $25 one at a sports store. Some have better handles/grips but the mechanics are about the same. We had a ballbearing (Home Depot) wheel and rod running through at a gym I frst started to train at..
A negative may be that some may experience lower back pain after use for awhile; nerve, disc, muscle imbalance, etc.. Something that anyone will have to discover themselves . Might suggest Romanian deadlifts if there is a weaker muscle imbalance of the lower back, because the ab's are a potential very strong muscle group and that strength balance can be very important.
Good Luck.