I can speak from experience with the Wonder Wedge (now Power V grip). It increases the size of the angled 2 and 4 bevels. This gives the continental and semi-western grips a bigger, more stable bevel. The #3 bevel gets tiny so if you use an eastern FH it probably will be a negative for you.
I originally tried it because my daughter was having a problem with her grip rotating from conti to eastern when she served. She didn't like it at first but after a few days she got used to it. It did solve the problem of the racquet rotating in her hand. She loves it now. After using it for a couple of years now she hates hitting with a racquet that doesn't have one.
I gave it a try as I would often use one of her racquets when we played. I also hit with a SW forehand so it was an easy transition. I really like the feel of the racquet with it.
I have since stopped using it. Not because I didn't like it, but because it forces you to hit on the same side of the racquet for all your shots. I use textured poly (MSV Focus Hex). What I found is that the hitting side would be worn smooth and the side with the Wonder Wedge would still be rough and like new.
I wouldn't call it a "gimmick". It really just gives you a more stable feeling for certain grips. Some may not like it. For others it really helps their confidence.