We have one of these locally and I've used it to get rid of unwanted stuff. In general I'm a bit negative on the idea actually - the hyperlocal aspect of it makes it susceptible to a sort of privilege bias. Rich areas will gift each other nice clothes, hifi equipment, computers; people living in poor areas, who might have far more need for this stuff, will be less likely to benefit (because there will be both less excess goods and more demand).

For this reason I just give anything of remote value to charity shops instead - they have the geographic distribution to spread donations around, and it means a charity gets direct monetary value as well.