Read blut's first post. The reason guys go real conservative with free weights, after being like animals on the machines, is because they have to. Machines isolate the specific muscle - Hammer strength chest press for example. You sit and push the weight away from your body. The basic muscle used is your chest (and a bit of shoulders). On a free weight bench press, chest, lats, shoulders, even the quads get hit. Also, all of the little stabilizer muscles get hit on free weights. You'll never know you have these muscles if all you do is machines.
If you want a bit more defenition on a particular body part, machines will really help. If you're looking for size and strength gains, then free weights are the only way to go.