I will agree that training aids are a dime-a-dozen. Every year or so, a guy comes out with a new training tool that is "the best ever." It may be for pitching or it may be for hitting. Most of them end up sitting in a corner somewhere. I have rarely seen coaches actually use training aids that are purchased. Some may get used.
Here is my sage advice. Skip the 'training aids". You don't really need something to tell you that you are doing something wrong or right. You need something, better yet, someone, to teach you how to do something right. I think of the Swing Away. It was supposed to be A great swing trainer, but in reality, it only told you that you did it wrongly or correctly. It was no trainer, it was a feed back device.
Get a good coach to teach you how to do things correctly. He should show you what it feels like to do it correctly. Then he can help you with using various feedback devices.
Now, what constitutes a good coach is another thread in and of itself.