Kettlebells are one of the best total body training tools available. With proper technique and weight they can add a whole new dimension to your training program. Women can normally start with a 8kg bell then progress into a 12kg bell. I have seen some women snatch a 20kg wich was pretty impressive. Men can generally start with a 16k then progress into a 20kg, 24kg, or 32kg bell.
Start with basic 2 handed swings then progress into 1 handed swings, there are so many combos that you can do, the possibilities are endless. It will be tempting to want to start with a bell that’s too heavy but work until technical failure.
As for the cardio issue, I think the mindset is that kettlebells also drive heart-rate and build up total body endurance. They are great because it is similiar to interval work as a runner. For some, cardio is boring, here is a great way to drive heart rate and keep yourself from getting bored.