Beware the sound byte, like this recent New York Times headline Turning to Kettlebells to Ease Back Pain.
The actual study compares Danish workers who did 2-3 [PROFESSIONALLY SUPERVISED, PROGRESSIVE INTERVAL TRAINING] sessions with kettlebells a week to those who were "encouraged to exercise" and concludes that kettlebells reduced neck, shoulder and low back pain.
The same effect from strength training is well known, and the Times article links to a large scale study that shows resistance training helps reduce musculoskeletal pain.
Mentioned in the article, but missing from the headline: "kettlebells can be difficult to control, it's important to learn proper form from a certified instructor." Kettlebells may be an effective part of a fitness program for some people, but done incorrectly there is a high risk of injury, especially to lower back and other gym members. That's why many gyms don't allow members to use kettlebells without supervision.
Like any fitness program, the first step with kettlebells should be an assessment. To avoid risk of injury - especially to the lower back - you need to have sufficient flexibility through the hips and back. in many cases, flexibility training or preconditioning without kettlebells may be advisable.
Similar or better results may be obtained without use of kettlebells, using traditional strength training programs that entail less risk and have more scientific research behind them.