Inspired both by Matt's post on Music. Marketing. Management. and our VP and head of New Buzzness Development Carrie's recent experiences with Qwitter, I wanted to throw in my own two-cents about the application.
For those who are hearing about Qwitter for the first time, Qwitter is a tool that notifies you when somebody "unfollows" your Twitter feed, along with the last update they received from your account. This way, if you notice a sudden mass exodus of followers who all "unfollowed" you after a particular one of your updates, you can likely attribute it to that comment more than sheer coincidence.
So should you use Qwitter? I believe the answer really depends on your specific goals for Twitter. If you have a very clear target demographic that you are trying to reach in order to sell more of your product, Qwitter can be useful in helping you figure out what information your target is most interested in. Maybe they are looking for updates on discounts more than they care about your product's various uses. That's not to say that you should completely avoid posting relevant information (even if it's less interesting to some customers), but rather you'll have an idea of which topics you might want to address more or less frequently.
However, there is an inherent danger in using Qwitter - namely, that you will begin to take your "unfollows" personally, to the point of losing your branding in favor of more followers. Just as there's a tendency to measure the initial success of one's Facebook page based on numbers of fans, it's easy to worry about your number of followers on Twitter. Whether it's Twitter or Facebook, I tend to take quality over quantity - it's more important that you are reaching and serving those who are legitimately interested in your brand and your products than to reach great numbers of people who might simply be looking for more followers themselves. You should certainly keep your customers in mind when updating your Twitter feed, but if you find yourself censoring your posts on a regular basis or completely reorganizing your Twitter goals in response to "unfollows," it might be time to quit the Qwitter.