just to re-iterate the above points

Free weights work all your muscle groups and will increase muscle control and overall strength if you want to use it for sport or work, i.e. be stronger for rugby, boxing, etc.

Like Phil said machines will give you perfect form and movement working a specific group. This is good if you're more interested in repairing an injury or want to increase muscle size fast and easily

if you want to increase strength and ability - free weights
if you want to get big but aren't too fussed about ability, form, etc - Machines