I do a variety of workouts with kettlebells. I feel that you can use them both for strength and cardio workouts. If you want a strength workout, use heavier bells and slow down the movements. If you want a cardio workout, use lighter bells and do more compound movements.
I still tend to do a day of traditional weight training because I like it but otherwise I do lots of Crossfit type workouts that use a variety of equipment (med balls, jump rope, sandbags and kettlebells). I have not lost any strength. In fact, I feel that I am stronger.