I bought an Eye Coach for my wife and I think it's a valid tool for beginners to find their hitting strike zone. It complements time on the courts without having to chase balls.
Is it good for the intermediate to advanced? No... not in my opinion.
I bought mine relatively cheap because they were selling them at the Family Circle Cup in Charleston. The inventor is a pro there at Daniel's Island.
Volkl C10 Pro w Pro's Pro Blackout 47-55lb. Still rocking my Klippermate.