I will give my view on the Smith Machine. It is a very useful and effective device but there are some caveats:
1) People new to lifting or with under 5 years of total lifting experience should not use this device. Developing proper form and stabilizer muscles is critical for the foundation of a strong and symmetrical physique. After 5 years of dedicated lifting, you are in a more advanced stage and have done the majority of your fast muscle growth (if you stay natural). Therefore, the Smith Machine is a great addition to workouts especially shoulders and legs.
2) Some Smith Machines are better than others. I prefer ones that are not counter-balanced (which are harder to find these days) and some are smoother in their operation.
3) When you are older and train alone, the Smith Machine is great for safe, effective compound exercises. I use it exclusively for deep squats. When done properly, you will get all the benefits of regular squats without the need of a spotter. The key is not to use the Smith Machine for everything so you continue to build and strengthen your stabilizer muscles. I use the Smith Machine for deep squats and occassionally for shoulder presses (front and back) only. I find it does not work well for me for chest, back, or other muscle groups.
4) If your goal is powerlifting, you must avoid the Smith Machine at all costs. If your goal is bodybuilding or improving your physique, the Smith Machine is a fine alternative for many free-bar exercises. The Smith Machine, however, is like a meal supplement - it is meant to be in addition to free weight exercises not a replacement for them.
5) Everyone is different in how they respond to certain exercises and machines. I have found over the years that my legs and shoulders respond well to training with the Smith Machine but the rest of my body does not. I do not have the knee, back, or shoulder problems some people experience when using this device. More often than not, it is due to improper form or their specific biomechanics which makes a fixed-plane device ineffective.
The bottom line is that lifiting is a journey. Over time you will find exercises, routines, and equipment that work best for you. THis whole debate over the Smith Machine as either a Godsend or the Devil's handywork is crazy. After 5 years of lifting with free weights, if it works use it for bodybuilding. If it doesn't work for you, don't use it.
John