Ab Circuits is a mixed bag. I like the medicine ball workout (solo, not partner) and the Pilates workout. I have tried a couple of the others once or twice, and they were fine. I need to try those again. Have tried the stability ball workout---I don't usually dig those (exception: KPJ core/abs segment)
I really like CoreMax, particularly the medicine/stability ball workout (#3).
Ab Hits remains my favorite of Cathe's Ab compililations/DVDs. I prefer straightforward, traditional ab work, so that's probably a factor.
One thing to keep in mind: since I've been doing the medicine ball ab workouts (plus the one on Susan Chung's Rapid Fire), my core/abs have gotten really strong. In addition, I've been religiously doing a couple kickbox (with weighted gloves) workouts, plus heavy bag workouts, each week.