Coffee

Applying it directly to plants or mixed into potting soil has very mixed support. It's decomposing cellulose, like any other plant, so it's unlikely to be that bad in any application. Mostly it seems like well meaning coffee enthusiasts recommend it, while mixing grounds into compost appears to be widely recommended even by non-enthusiasts.

The thing I avoid like the plague is getting grounds down the drain. Over time they'll clog drains, collect in pipes slowing flow, etc. and generally be a pain. So I'm careful to get as much out before rinsing as possible without going crazy. I also do not use it in soaps, because soap film plus coffee grounds in your drain is a perfect recipe for those clogged pipes.

I've been meaning to try using used grounds in a cooking rub- leaving them to dry and then mixing them in a spice rub or using them as the rub itself. I'd be interested to see if anybody's tried used versus fresh grounds, and whether there's a noticeable difference.