Here goes my opinion. Most ab exercise like sit ups and leg raises are exercising the psoas muscle as it's major mover. The actual abs people show in bodybuilding accounts for very little movement. They isometrically contract during many major exercises like deads and squats. Almost as a protective muscle. The range of motion is very small. When you do something as a situp it's approximately the first 20th of the movement has the abs at complete contraction. The rest of the movement is the psoas and other hip flexors. Weighted crunches is a great ab exercise but most don't understand how to do it to activate the abs. Most just bend their neck instead of lifting the upper back off the floor bringing the sternum to the pelvis region. Doing crunches sitting opposite in a pulldown machine holding a V handle behind the head crunching down is a great ab exercise.
Getting back to the ab wheel. The abs have to contract hard doing the movement or it will be impossible without sagging. The first time I tried the ab wheel thinking it was a gimmick left my abs incredibly sore the next day. I hardly experience any major soreness but wow I felt it in my abs. I think the way to go is to use a pad for your knees and roll out as far as you can. Throw in some diagonal wheel outs too. When you get good you can proceed from the standing position. Realize too it's not without it's risks. If you have any hernia problems this is going to be a real problem. It puts a real strain on the abs. Second it can be hard on those that have shoulder issues.
