I use twitter, though I don't post much. However when I have posted and it's been a moan about some company or other, they've been very quick to contact me and get it sorted.

So, I'd say it's useful but only for getting decent service out of companies that are otherwise terrified of getting a bad rep online. (Virgin Media, Royal Mail, a couple of others too)

(Incidentally, this post is flagged as unsuitable for minors. Why?)