My training has just naturally migrated to machines over the past year. I wish I would have known how good they were years ago.
Machines Vs Free Weights
Let's break this down...
Gym Machines
Here is...
1) The Value
They allow greater loading on the Primary Muscles in a Movement.
The limiting factor in a Free Weight Movement is the Smaller, Weaker Stabilizer Muscle are called into play.
Thus, the Primary Muscles are never completely OverLoad, which is one of the keys for Hypertrophy and Strength, as well.
"When The Back Says No And The Legs Say Go"
Hollie Evette, a National Rated Powerlifter and one of best Analytical Strength Coaches in the game wrote this article decades ago.
Evette went into how the Legs were never completely OverLoaded in the Squat. The Limiting Factor was the Back/Trunk Stabilizer Muscles which gave out long before the Legs did.
The Solution
Evette's article identified the problem and then provided Exercises that would resolve it. He prescribed Exercises that minimized the Stabilizers, placing the workload on the Legs with Modified Free Weight Movements.
With that in mind, Gym Machines and Exercises that minimize and take Stabilizer out of the equation elicit greater results for the Primary Muscle.
One method for individuals who have a Power Rack is...
Scrape The Rack Exercises
This method essentially turns a Power Rack into a Smith Machine; placing a greater OverLoad on the Primary Muscle and minimizing Stabilizer Muscles.
The Limit to the Scape The Rack Movements is your imagination.
2) The Downside
The benefit of Machines is also one of its Downsides.
Individual who do not incorporate Exercises that Develop Strength in the Stabilizer Muscle cannot perform optimally with Free Weight Exercises.
Free Weight Exercise are effective at training the Stabilizer Muscles; especially Unilateral Training and Dumbbell Exercises that require more involvement than when performing a Movement with a Barbell.
Summary
As Jeff noted, Machine definitely are effective for Hypertrophy and Strength Development.