I was explaining kettlebells to my Dh last night. Even with the basic moves like the swing, clean and press and snatch (*snicker* - couldn't they have come up with a different name?!) you really work a lot of different muscles. If you take the clean and press versus the military press - you start with the kettle bell on the ground and "rack it" (pull it straight up and let it roll around your wrist until your standing with your hand on your chest and the bell to the outside, then you push from that position into a shoulder press and back to rack, then back to the ground. (to see it click here -
http://www.youtube.com/watch?v=se6MmJkx2h0&videos=Eb5ZiwoJkyM&playnext_from=TL&playnext=1) In that one move, you work your quads and hams from the squat, your core from stabilizing a heavy moving weight, your lats from the press movement, and your shoulders. A military press works mainly the shoulders (yes some core from holding the weight above your head and pushing up, but mainly shoulders). And swings and snatches work the quads, hams and core like nothing else (and I mean the entire core - not just the abs)
Kettlebells also have a much higher cardio factor than traditional weight workouts. You can do a 45 minute Kettlebell workout and burn as many calories as you would from IMax 2. Plus, they are easy to fit in to so many different workouts to up the intensity without upping the impact. Knees or feet bothering you? Do IMax 2 and sub swings for the the interval blasts. You won't reduce the intensity at all!
You get the real benefits when you really get the moves down and start to up the weight. I did a ladder of snatches (to see a snatch look here -
http://www.youtube.com/watch?v=98z6zb9Mli4) I started with 10 snatches left and right, then 12, then 14 ... up to 20 - with my 20 pound bell - only did them with the 15 before. Man, my core (the entire mid section) is feeling it today. My legs are also feeling it as is my upper back, etc. And this was in a 25 minute workout.
Kettlebells are not for everyone and they do have a learning curve to get the moves down right to avoid injury. I would recommend finding a local gym who specializes in kettlebells or a really good tutorial like "The Kettlebell Way" to get started.