Just like it is important to challenge and confuse your cardiovascular system by doing interval training or using fartlek techniques, you can increase the benefits of weight training by varying the tempo of your reps.
While very slow repetitions challenge the endurance strength of the muscle, it is important to incorporate a variety of tempos to your workout, including medium speeds as well as super slow. You can also lift slowly and recover quickly for a set and then reverse by making the return much slower. Using these varying tempos challenges the muscles with different rates of tension, increasing the benefits of strength training.
To track your tempo count while you lift. For example, use a steady count of four to complete a bicep curl, and a count of one to lower the weight (not letting the arm flop). For your second set, or the second half of your first set, reverse and bicep curl on one count and lower for four counts. Tempo variations work well with both free weights or machines.
Remember, variety is the spice of life!