According to other online fora coffee grounds are a wonderful addition to bonsai soil, or awful, or so so. For those who think wonderful, they recommend adding directly to the soil mix, sprinkling on top of an already potted tree, or composting first, for a month or a year.

In other words, there is support for just about every answer.

Does anyone here have experience with coffee grounds? We toss three or four cups worth every day and it would be nice to do something useful with them.

Ivan, the main problem with coffee grounds is that it creates an impermeable layer on the soil, and watering gets compromised.
When I use, I sprinkle it in some parts of the soil, but never cover the soil with them.

Ivan Mann wrote: Does anyone here have experience with coffee grounds? We toss three or four cups worth every day and it would be nice to do something useful with them.

Off topic.,.. but:

I saved the rests of candles, melted them, mixed them in a milk carton with coffee grounds. Got them out of the cartons once they cooled off and made a nice fire.

Auk wrote: I saved the rests of candles, melted them, mixed them in a milk carton with coffee grounds. Got them out of the cartons once they cooled off and made a nice fire.

I'll surely try it Auk, thanks for the tip!

I should have asked my friend Dr. Google about coffee grounds before I asked here. Dr. Google has conflicting opinions (of course). Generally, he says used coffee grounds don't affect PH, and don't add nitrogen to the soil but do help compost develop nitrogen. They do help with water retention and may possibly repel cats and other animals. They do help soil retain water.
So, I think the answer to the question is use them as organic material, remembering what Clicio said and don't pour them on the soil at one place, but instead sprinkle them. Or, mix them in with the substrate as general organic material.

I used one cup a week, but then my plant began to dry out, so be careful.