1) They are portable: If you have kettlebells, you can get your workout wherever you are going to be. That is so valuable if you take your fitness seriously. For most people it is not the quality of the workouts, but the consistency.
2) They are athletic: Look at people using kettlebells. Look at people on a treadmill. What looks more athletic?
3) Adjustable leverage: Because the kettlebell is asymetrical, you can change the leverage of the kettlebell by changing how you grip it. Take presses: Grip it by the bell (waiter's press), grip it by the handle (bottoms up press). They all work your body different, JUST LIKE OBJECTS IN REAL LIFE. Try doing that with a barbell or dumbbell.
4) Enable High Rep Ballistics: Because the kettlebell can be swung between the legs comfortably, you can do high reps ballistics with them. If you are interested in fat loss, this is critical. Swings, snatches, and cleans can be done for hundreds of reps. HIIT, and the fat loss and cardiovascular conditioning that comes with them, are so much better with kettlebells.
5) Cheap: Kettlebells, when you compare them long term with a treadmill or your gym membership, are a ridiculous bargain. For the average female, for $400-$500, you can get enough kettlebells to last you for the rest of your life. That's right, no other investment in your fitness. That does not even buy you 1 year of a gym membership. Don't even think of getting a treadmill for that little. For a home, a treadmill will easily cost between $2000 and $4000.
6) Versatility: You want to burn fat, get stronger, build muscle, and improve joint mobility. Kettlebell deliver on all counts, often in the same workout. In my kettlebell boot camps and workshops, all of these are covered.
7) Durable: Kettlebells last forever. No moving parts.
8) Extensible: Kettlebells workouts can be simple or as complex as you want them to be. Need a simple workout, the Enter the Kettlebell Program Minimum is where it at. Want to build muscle, I recommend Return of the Kettlebell. Fat loss, Kettlebell Burn will do the trick. Whatever you need is whatever you get.
In summary, if you have resolved to get in shape in 2011, take the leap and dive into kettlebell training.