I see nothing wrong with kettlebells if you are working out at home and need as little equipment as possible. 2 kettlebells, one heavy and one not as heavy, thats all you would need.
But if you are using them at the gym it doesnt make any sense when there is other superior equipment available.
Furthermore, and I may be wrong on this, but havent studies shown that cardio combined with resistance training is inferior for building muscle? So really kettlebell routines dont seem too usefull except for weight loss and muscle endurance (which may be some peoples main goal)