I agree: not sure about plants actually liking [too much] coffee in their soil, but mixed in with the compost it's fine. Years ago I read that roses like coffee as a kind of fertilizer, but this didn't come from a horticultural source, and I haven't been able to verify it.
Regarding slugs: it does work very well as a deterrent / like a barrier: we surround either individual plants, or an entire bed with a 2 - 4 cm wide band of our coffee dregs, and have not seen a single slug brave it across. Our neighbours swear by it, too.
I think it's as good as these slug pellets, which you have to sprinkle meticulously and repeatedly around your plants, but a good deal cheaper (depending on your source)
My pot-plants in the garden get a band of coffee sludge around them, too - it might not be too pleasant on the eyes, esp. with dainty flowers, but does the trick.