To put it simply, weight machines are a waste of time.
Now, before you start breathing fire on your keyboard and pledging your allegiance to the United States of Nautilus, I urge you to hear me out. Many of the reasons machines are praised by the larger population are the exact reasons why you should not be using them, and in fact, they are hurting your own physical development.
Where do I come off with the audacity to make such a bold claim? Well, you could probably blame it on my upbringing as a personal trainer. Over the past year, I've been lucky enough to have worked at two of the best gyms in the United States (and yes, that opinion is both objective and subjective). While the two gyms differ in their main audience, they are both operated under the goal of getting everyone to move better before embarking down any other type of path. One of the other main similarities wasn't as much in what they both have, but in the absence of weight machines.
So, why are weight machines so terrible? Taking a larger view, when you practice a certain movement on a machine, you will get better at that movement. In fact, if you work out on a machine, you'll probably be able to lift more weight on that machine in short amount of time....but, that's it! You get very good at using that machine, but there's no carryover to using that strength in everyday life. Isn't that the reason you're working out in the first place? (Ok, maybe you want that whole "look good nekkid" thing too..)
One of the great (or not so great) aspects of strength and conditioning is the element of specificity. Put simply, if you only did deadlifts and never benched, pulling heavier weights in a deadlift will not carryover to your bench total. If we take a machine such as a leg press, you'll get very good at having your back against a board and pressing your feet against something in the air, but if you try to tackle a normal squat with the same weight, you'll crumple like a piece of paper (believe me, I've seen it).
Next, machines are often praised for their ability to give you more "stability" and "control" when you use them. In fact, it's really just the opposite. Performing exercises on machines give you a fall sense of strength, and it develops the wrong parts of your body. If we take the leg press as an example, you train your prime movers (think big muscles like your glutes), but it ignores your stabilizers (think hip flexors, adductors, etc). Without this additional development that you'd receive from an actual squat, you're training your body to fire a cannon from a canoe.
Finally, the "isolation training" that many machines offer hinders your development from actually getting strong. What do I mean? Anytime you're lifting a weight, you need as much tension from your head to your toes as possible. Contracting your glutes and core on something like an overhead press or a bench press is going to make you much stronger, and allow you to lift a lot more weight. When you go through the motions without this necessary stiffness, you're allowing yourself to "leak" energy and reducing that amount of weight you can actually lift. When done correctly, every exercise is a full body exercise!
In general, I tend to give everyone the advice that when they head to the gym, they should be doing exactly the opposite of what everyone else is doing. We all like to do what's comfortable, which explains why you see tons of people walking on treadmills, on the elliptical, and using weight machines. In reality, we all really need the opposite.