AT HOME WORKOUT: Time saver or time waster?

When I got to college I went from a girl who could work out for 4 hours a day, to one who couldn't find the time to sleep for 4 hours a day.  Freshman year I was in the dorms, was close to our rec center and had a significantly lesser class load.  This year, as studio work and class work piles up, a 30 minute walk to the rec just is not conducive to my schedule, and due to a recent knee injury so I have started working out at home.  The Nike Training Club, My Fitness Pal, and Jillian Michaels have become the new staples in my workout routine.  How do you keep fit with a busy schedule?