According to this presentation, 67% of US Twitter users are more likely to buy brands they follow. I have found that tweeting as a brand, a professional service firm (rather than as an individual), is difficult because, let's face it, the tweets are often mostly promotional which makes them boring. I don't know many, say, accounting firms who are tweeting as the firm that have compelling content in their timelines. I follow several of them so I speak from that experience.
A key for professional service firms tweeting as brands is to have individual evangelists inside the firm tweeting interesting stuff and/or let the firm have a personality. Either way, it's a hurdle for many professional services firm.