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My feeling is that it is both somewhat useful and "huh, that's neat". It is interesting, and potentially valuable to be able to gauge reaction to a new ad or ad campaign in real time. If twitter and a method for analyzing twitter traffic had existed in 1984, for example, I'm sure reaction would have been off the charts when the MacIntosh commercial aired during the Super Bowl. But in today's world, especially with viral marketing, it can take some time for an idea to implant itself into the mass consciousness. Having some experience with Fortune 500 companies, the concern this generates is related to the VERY short amount of foresight many American companies have. Everything is judged in 90 day cycles, because of earning reports, and anything that isn't fully realized in 90 days can result in investors panicking because they hear a rant on a morning business program about a missed earnings projection, even if those investors know essentially nothing about the core business and care even less about long-term strategic planning.
That said, if there is a high cost advertising campaign deployed, it makes sense to have some metric that indicates effectiveness, especially if ad buys are ongoing. What I wonder about with DataSift is if the quality of the twitter traffic is measured, and not just the quantity. A good example of this would be the Groupon ad that aired during the Super Bowl this year, where it was widely perceived that Groupon was belittling the plight of Tibetans. I suspect that there was heavy traffic about the ad, but a high percentage of it was negative. So theoretically, an advert campaign that was found to be highly offensive could be mistakenly seen as highly effective. I concede that some would say that there is no bad publicity, and that if people are talking (or tweeting) about your company, it is inherently positive. If DataSift has accounted for this, though, I would think it could be a useful service.