Which gives you the fastest results with building muscle, Machines or Free Weights? Answer: It depends on the manufacturer who made the machine. Companies like Nautilus, Free Motion, and Medx build their machines to follow the proper functional strength curve of a muscle. This means that the resistance varies over the course of the rep, getting harder as the muscle contracts. For example when you're doing a bicep curl, the resistance is as such that at the start of the movement the weight feels lighter and as you curl the weight it feels heavier and heaviest at peak contraction. Test it out yourself next time you're at the gym, do a set on a Non-Nautilus/Medx/Free Motion machine and then do a set on one. You'll feel a world of difference!
The flaw with free weights is that there are very few exercises that puts the resistance at peak contraction of the muscle. If you take that same bicep curl for example, the only point the weight is the heaviest is when your lower arm is parallel to the ground. In that position, gravity is at its greatest force. This is basically in the middle of the movement not at the peak contraction where it needs to be. At the start and at the end of a free weight bicep curl the weight is actually moving mostly horizontally which is completely ineffective.
Don't be misled and think that I don't recommend free weights. They are a far superior tool than most of the machines out there except Nautilus, Medx, and Free Motion. An incredible amount of muscle can be built with a few very basic free weight exercises done 3x's per week. With the right type of machines, you can get the same results in a matter of months.