I think circuit training is a highly productive training model.Most people don't understand how to use it effectively.
You can still strength train while doing circuits, but you have to set your program up a certain way.Limit yourself to just a handful of compound lifts that, when combined, hit all the major muscle groups.An example would be:
Front squat
Incline Bench press
Stiff-legged deadlift
Pull-up
Do 5 sets of 3-5 reps with a moderately heavy weight for each lift.Use a one minute rest interval between exercises.The alternating between upper and lower body movements will keep the body from cooling down too much.You can probably complete each circuit in about 5 minutes or 25 minutes for all circuits.
For cardio, interval training goes well with
resistance training.YOu could either do a short interval workout following the circuit routine, or a slightly longer, more intense interval session the following day (depending on your goals and recovery ability).
Circuits work well for athletes too, who, because they need to focus the majority of their time on skills development, must get there strength and conditioning out of the way quickly.If you are focusing more on conditioning and power development, you can add some more dynamic exercises into your circuits like burpees and medicine ball tosses or throws and keep the rest intervals between exercises relatively short.You can also do active rest between exercises such as 30 seconds of fast pace jump rope.It all depends on what you are trying to accomplish.
Bodybuilders can also benefit from a form of circuit training called PHA or peripheral heart action training.Training legend and bodybuilder from the 50's, Bob Gadja pioneered this system.Do a search and you should come up with some interesting history.