I'm definitely a big Kelly fan! My top 3 favorite KCM DVDs are: 1) 30 Mins to Fitness:Weights; 2) 30 Mins to Fitness: Cardio Blast; and 3) Split Sessions. I always feel like I get a really great and challenging workout with these DVDs.
All 3 of these workouts have someone doing an intermediate option and someone doing an advanced option (Kelly does the advanced, too).
With the weight workouts (#1 and #3 on my list), you make it as challenging as you want by going as heavy as you can. With Cardio Blast, you can follow the higher impact advanced options shown by Kelly and another background exerciser.