Most of my life I have been a tinkerer and once I played with idea of coffee grounds as a potting medium. I found the coarse grounds from a Walmart house brand was worth a second try but soon failed to be worth the effort. The grounds from Starbucks had a lot of espresso grounds in them and just formed an unmanageable ball. Final results; I think a fairly large amount of the cheap grounds 10-20% perhaps, could be mixed in with a potting soil.

There is no benefit to adding coffee grounds to indoor potting mixes. It might be used when composting for outdoor gardens but compost is not recommended for indoor use.

To me the main benefit is not trotting outside during the winter... on nasty cold days I just dump my grounds into a convenient plant container. I don't mix them in, just spread on top. I think they deter fungus gnats, although I haven't tested that rigorously. Also, some of my containers have worms living in them, and they enjoy the grounds (so some do get worked into the soil).

For some plants coffee grounds actually inhibit growth if not composted first, if you add too much. They have been tested as a herbicide for this reason although the inhibitory effect is reported to gradually wear off.