dclegg:
I do it quite a lot. In some instances, it can be far quicker than more traditional customer service avenues. Especially if call centres are involved.
I find that many NZ companies have harnessed Twitter quite well to turn negative experiences into positive outcomes. As a customer, I think it's key to keep your criticism as constructive as possible.
It is also important to see whether this will actually be an effective communication medium, by browsing their timeline and seeing if they actually respond to to tweets. There are still quite a lot of companies that simply use social media as another channel for one way advertisements.
People who whine and biartch, happen. People manning Social media are also people, dont shoot the messenger. The issue could be slackness, bad process, bad luck, but in todays world, many try to make it better as there is so much competition, its often about service. But sh&t happens. Complain, query, but be nice.