A kettlebell is just one small piece of equipment that allows you to have a rather complete workout. Once progressing you may need to increase the weight, but still, something that you can easily do at home. There are plenty of exercises available, online and in books. Just my personal impression but I feel I have exercised well after a session.

Obviously, proper weight training in a gym will bring better results but I don't think that this is necessary if all you care is staying healthy and in shape. I just mix it with other sports like running.