I recall reading a research article recently that stated, in summary, that the benefits of circuit training - which are considerable - depend upon your general training and fitness level. For more beginning exercisers, circuit-style training can promote and improve strength and mass development as well as cardiovascular muscle endurance; for more trained exercisers, the benefits are more maintenance-oriented, again for cardiovascular fitness and muscle conditioning.
I think a lot has to do with that, AND with the type of circuit workout you're doing - whether it's cardio interspersed with total-body resistance training, cardio interspersed with strictly lower-body or upper-body training, or a pure circuit that is strictly resistance training (which is where circuit workouts got their name and start). Personally, for circuit work I exclusively do cardio + lower body resistance workouts because I like to keep the cardio challenge going. But there are so many options, especially Cathe options, that you can't help but find a circuit routine that works for you.
Another part of the benefit, for me anyway, is that I too love them, and I'm far more apt to be consistent with a workout style I love than one that I don't. In fact, I love 'em so much I've kind of done them to death the past couple of years or so, and may have to take a couple three weeks off and do strictly steady-state and unweighted interval work.
A-Jock
