I conducted a bit of (totally unscientific) research on this last spring/summer. It works, but only up to a point...

The grounds have to be piled on thickly around the plant to be protected.

You must refresh the grounds at least every other day for them to have any effect, ideally every day, because once they dry out/get a soaking in the rain it's no use.

It had no effect on taste/appearance of plants I used it around.

All in all, not really worth the hassle unless you have absolutely mountains of grounds to use up, like if you owned a coffee shop or something. There are other organic mulches you could use to try and reduce slugs, but there must be hundreds of methods and hundreds of opinions on what works and what doesn't!

This year I will be mostly experimenting with egg shells and copper in various forms.

Good luck! Bob