In general, you don't want to use any uncomposted material as a potting mix. In some cases, limited amounts may be included in a potting mix, but the majority should be something else. I don't know what the pH of coffee grounds is, but it could be way too acidic for good plant growth. If you start composting them now, they should be usable by next year, maybe sooner. Having said that, you can always try growing something in a side-by-side trial and see how it compares to a standard potting mix.

Good suggestion botanybob - what I have observed is that errant japanese maple seedlings have sprouted - univited - in the coffee grinds left out in the open........so the ph maybe isn't too high for them!

Coffee grounds are sup-po9sed to be acidic, but I have not found any listing of what the axctual pH is. I would suspect that, like Oak leaves and Pine Needles, you would need to add tons of coffee grounds to your soil before any significant change in soil pH would occur, so that they are acidic would not be of concern to me. Starbucks saya the nutrients available from coffee grounds is
Primary Nutrients
Nitrogen 1.45%
Phosphorus ND ug/g
Potassium 1204 ug/g

Secondary Nutrients
Calcium 389 ug/g
Magnesium 448 ug/g
Sulfur high ug/g

Notes:

ND = indicates sample is below detection limit
ug/g = microgram / gram

Used coffee grounds will be fairly close to neutral. Raw grounds can be quite acidic.

I don't have an answer for you, but your post brought to mind a peculiar habit my husband's grandmother has. She regularly dumps her used grounds outside her back door each morning, right on top of the plants growing around her back porch. I always thought it was because she didn't want them in her trash can...now, after reading your post, I have an idea why she does that.
The grounds don't seem to harm the plants - they look fabulous. Lush and green. But she has plants that are acid-loving, if I remember correctly.

RENEE

As a follow-up to Renee's point, my mom used to do the same thing, telling me as a child that coffee was like a fertilizer - flowers did great. I don't think used grounds will hurt the plants - if used as a soil additive. Doubtful that the grounds should be the entire soil, however. Add the extra to your compost.

Caution: Not all plants like the high nitrogen content of used coffee grounds. It can kill heaths and heathers, for example. However, hostas love coffee grounds and slugs don't.

Thoughts from some posters who have tried grounds as a substitute for other potting mediums here -

Here is a link that might be useful: Coffee ground thread

Should you use used coffee grinds or dry unused ones?

I have tried putting the grinds around my hostas as a slug annoyance. Apparently the critters dont like the multi-mini-terrain for their slimy movement.

It really isn't the coffee grounds themselves that are so beneficial to your plants but who they invite to the picnic. Earthworms *love* coffee grounds and will travel to get some....earthworms are our friends ; )

I kept a bucket of used coffee grounds indoors and they got really moldy. I wouldn't recommend using them alone.

I dump mine around my wifes Roses along with banana peels. They seem to like them just fine. Now mind you I only dump about 4 tbs of grounds a week or less even as I dont always drink coffee.

I've been adding used coffee grounds to my Rudbeckias (Moreno and Cherry Brandy) and they are going nuts and getting huge. I like to think that they are looking great due to my skill and good use of recycled matter, but I think it may just be that they grow quickly around here.