I know I'm going to get flamed for this ....
I just got one of these for Father's day and spent the last two days using it and having some friends hit with it. I think it's a really cool tool. I can tell instantly when my timing or posture is off simply by where the ball goes. The Sweet Spotter is like putting your strokes under a microscope. It meakes you focusas there is a very small margin for error. Technique flaws that may have been missed with any size racquet are glaringly obvious with the Sweet Spotter. It's not a cure all and it's not perfect but it is a really good tool.
I haven't figured out 100% what a top-spin shot should look like with the SS, so when I transfer to a raqcuet, I have the same timing... I feel I'm getting close.
I have a friend who in is earlier years was an open-level player. He also hits pretty flat. He picked up the Sweet Spotter and was just wailing on the ball. He's always been known as a clean hitter. It really showed when he picked up the Sweet Spotter. (I really hate him sometimes)
The only issues I have is it's too expensive ($80-$100 feels right) and the grip feels a little funky to me, it's not easy to find the bevels.
It's a blast to hit with and I beleive it will help, especially on serves and volleys.