Been using then for 2 years now, during off seasons to strengthen my core. I use a combination of 12kg’s (for KB pyramid – 20 swings, 10 pressups, 20 swings, 9 pressups etc) and 16kgs for strength (12, presses, 12 lifts, lunges etc.)
Its good to get a DVD to observe the correct form as thats important, but lifting KB’s over a short burst of exercise will tone you (they increase your metabolic rate and workout all the core and supporting muscle as they are an “unsupported” weight), lose weight again through effort, and through the longer lasting 24-48 period that your muscles are using fuel to “fix” themselves.
Like all exercise its important to eat and hydrate properly to support your efforts to maximse the benefit.
Also, I paid £75 for 3 KB’s 2 years ago, and can workout at home any time I wish performing a workout for any muscle group – compare that gym membership….