It works great on my shop vac. I turn it down to reduce the noise.

I use this with my very powerful exhaust fan when doing construction to keep dust/fumes from entering other parts of the house. Depending on the size of the area im working in, my fan may just be too powerful, this fixes that problem very effectively.

The built in speed controller on my table router, a 3-1/4 HP Hitachi M12V, failed and a replacement controller was very very expensive. So rather then replace the internal controller I purchased the VIVOSUN controller and bypassed the internal controller on the router.
The product description nor any reviews talked about its used with a router, but it looks and is specified identically to other controllers that are used with routers. This unit sold through Amazon was also less expensive then the identical looking controller sold by the local import hardware store even with their standard 20% discount applied.
Using the controller with the router (with its internall controller bypassed) the Vivosun controller works great giving me a full range of speeds and to my ear hits the max RPM offered by the router.
One significant annoyance is the mounting provided. There is only one option to mount by a metal clip to hang it off something. THis is not ideal when mouting to a wall or side of a router box and the unit is not solidly locked in placed and will move and jiggle around from just hanging from its clip
CONCLUSION: Other then the mounting the unit works great and a much less expensive option then replacing the internal speed controller of the router.

I purchased this unit to control the speed of my die grinder when I use various size burr bits. It does exactly what it says it will do and works well. As a bonus I didn't think of... Lol it also acts as an extension cord to extend the reach of your die grinding capabilities. The future I wish they would remove is the ability to turn the unit on all the way or use the speed. In my application people I work with use the switch but leave the die grinder on so if I use it stand alone I plug it in and it automatically turns on which could be dangerous to some.

When I first tried using this, it just wouldn't do enough to slow down the drill I was using. The drill normally spins at 900 RPM and the boring tool I was using said not to exceed 450 RPM. I had read several revues referencing this problem and suggesting a way to adjust the speed control. After removing the back of the speed control, I found that the potentiometer was located on the underside of the circuit board (as it relates to the removable back on the controller. However, I found that the manufacturer had drilled a hole in the back side of the board so the adjustment "screw" could be accessed without further disassembly. I gave the "screw" a slight turn to the right and now it controls the drill perfectly through all the ranges from slowest to full speed. I have a very similar control that I purchased over 20 years ago that is still working perfectly. I am hoping that this one does the same.