Sometimes Repetition is Good

Whether you are doing a circuit using gym machines, weights at home, or just bodyweight training, circuit training is a great way to work multiple muscle groups without getting bored or overstrained.  It also adds a bit of cardio into an otherwise straight weights workout which will strengthen your endurance, cardiovascular system, and core muscles.  
The main focus in circuit training is continuous motion in conjunction with strength training.  While strength training is important for longevity and that nice toned/ripped look, straight strength in everyday life usually won’t cut it.  There is a big difference between being able to lift a heavy item and being able to continuously lift heavy items, such as in building or mechanic work.  The extra balance needed will also strengthen your core in a much more functional way than just straight ab exercises.  
Again, Jillian Michaels has a few really great circuit training type programs, including 30 Day Shred and Ripped In 30.  Insanity by Shaun T is a great one for the hardcore athlete; it’s a 60 day program, complete with calendar and tracking test so you can see your progress even if you can’t “feel” it.