I have recently been introduced to the world of kettlebells and my abs are thanking me for it! I thought that it couldn't really make that much of a difference in my workout, but it really has. I have both 5lb and 10lb kettlebells (GoFit 10-Pound Yellow Kettlebell with Vinyl Coating, Iron Core Training DVD and Exercise Booklet); the 5lbs for when I am learning a new workout so that I don't hurt myself and the 10lb for actual muscle building purposes. They are great because I use totally new workout routines and am working my muscles in different ways. Also, you are combining cardio with strength training which is the best kind of workout; you increase your metabolic rate significantly, so it truly is a fat burning workout. The bells are so different from regular dumbbell weights because they are unstable and it requires extra concentration and muscle strength to stabilize the bells, allowing you to use more muscles with each workout AND most often you are using your core muscles to aid in the stabilization. Many of the workouts are full head-to-toe workouts and utilize every muscle in your body, that focus on muscle movement and toning, so you don't bulk up (you can if you want to and use higher weights, but I like to tone).
Also, its fun, new to your body (so you burn more calories), and you are supposed to supplement your workout twice a week with kettlebells so it changes your routine. Kettlebell routines are easy to learn (Turkish get-ups are about the most challenging and even I could get them down quickly!), also it is low impact on your joints, and what is better than high intensity cardio that is low stress on your joints??
Give it a try and let me know what you think, they have changed my abs!!
Yours in Good Health