As somebody who regularly takes personal training sessions with different personal trainers (one for weights, one for boxing, one for swimming), I dare say that a good trainer can make a big difference and can be well worth the money.
Yes, I know how to perform almost all exercises with good technique, but I have made the experience that often when I hit the gym after a long and stressful working day I just perform the same basic exercises, go through the motions, while my mind is still at work.
Having a trainer structure my workouts, mix things up, push me to my limits and teach me new things is very helpful.
Does this mean, that everybody would benefit from a good trainer ? Maybe, maybe not. We are all different. But in my case (maybe because I managed to find very good trainers), I learn something new in every workout and I can definitely see improvements that I would not have if I only worked out on my own.