I think ab circuits is amazing! Watching Cathe do some of the moves truly inspires me to drop my jaw - one of the moves in particular is when she is sitting on the ground, cross-legged. She lifts herself up with her arms - then while she has lifted herself off the ground, she uncrosses her legs, straightens them out, and them crosses them again, and then lowers herself back to the ground...talking all the while! Amazing! The pilates abs, and the weights & plates are particularly good, I think. As for not getting a good ab burn...I'm not with ya on that. You must have some pretty good abs! I definitely find that I have room for improvement.
I like the medicine ball abs because it is short and do-able, and the abs - no equipment is also less difficult. I like that there is a variety so depending on how I am feeling I can do the workout that fits my mood...instead of just blowing off abs (which I am prone to do if it is "too hard" for me that day). Not good, I know, but I am being honest.