The only useful training aid.

Besides weighted clubs which have been a proven training aid for the last century, as Harvey Penick prescribed the use of the weighted club to properly reinforce muscles used in the golf swing movement.

There is maybe only one useful training aid on the market. "The impact bag"

At least 95% of golfers have their hands behind or on top of the golf ball at impact position. The correct position would be to have your hands in front of the golf ball at impact. That's where the impact bag comes handy as you can see and feel your true impact position. I'm not saying you should get one, I'm just saying next time you hit the range concentrate and try feeling where your hands were when your golf club touched the ball.

8/10 for the Impact Bag. You don't need one, but it could help.