I've been doing circuit training for about a month now. Three times/week, about 20 minutes a session, using Smith equipment.
The key in my case is full-body workouts each time, with progressively increasing weight/intensity. 1 set each exercise per muscle group 8 to 12 reps max....
And it's working. I've noticed I've burned off some fat, and my strength is returning. I'm moving on to another program next week that's focusing on body weight exercises (chinups, dips, etc.) and heavy lifting (deadlifts, squats, etc.) for mass building.
I'm also a fan of compound over isolated exercises...abbreviated workouts over "marathons."
Circuit training is good for conditioning/toning/muscular endurance. Keep at it!
How's that for a pity post?