I do cardio at least 3 days a week. Sometimes it's stairmaster and treadmill, sometimes I just walk around the city for a couple of hours.
When I do frequent cardio, I feel like my joints stay fresher. It also increases my metabolism which is helpful for staying lean. Another thing that all these young bulkers should realize is that when you do cardio, increasing your metabolism, you get hungrier, which makes you eat more, which facilitates weight gain. I know that sounds counterintuitive, but the more often I do cardio, the hungrier I am. And I mean that my appetite becomes insatiable.