Try it out, people are usually happily surprised to see this type of workout builds muscle mass. This workout is designed to provide good general functional fitness anaerobic and aerobic . Even if you are in really good shape you may find one of these workouts will suprise you ;)
Functional movements executed at high intensity have been proven to elicit a neuro endocrine response. "high-intensity intermittent training may improve both anaerobic and aerobic energy supplying systems significantly"
http://www.ncbi.nlm.nih.gov/entrez/quer ... t=Abstract
keep lifting if you like it, always good to mix stuff up, maybe try integrating some of your lifts with some of these circuits to increase the intensity..