Here a good article on why the Kettlebell Swing belongs in your fitness arsenal. I've never used kettlebells, but I do know one-arm DB-swings are fantastic. The swing forces you to maintain tension throughout the core (unless you want to throw your back out), works the whole posterior chain, and makes you generate the power and snap from the hips (your arm really just serves as a pendulum, and your grip keeps the DB from flying away, rather than having to actively lift it). At least, that's the way they feel to me.
Update: As usual, the Crossfit folks come through with a video ("Kettlebell Swing" on the list).
