Hmm... when asking for a "success story," what would be the criteria to determine if it is successful?
I only ask because since we've released our own general Twitter feed, we've seen traffic increase to our site (as well as when we promoted the launch of this site) jump! Now, we're looking at additional ways to leverage the service for other internal workings (promoting our upcoming webinars, seminars, and other online tools/topics) but we're seeing some return on our investment in time.
Another success story was a simple test by a client of ours that we had setup for them (as a way to show how the service could be of benefit.) We setup a twitter account for a local music retailer and helped them understand how they could use the service to notify followers of any product promotionals or special sales events.
One idea, "48 hour Speed Sales." Essentially, when they only have one or two more of a particular product that they will no longer be carrying, they make an announcement through Twitter about the sale item, the sale price, and the location of the product (they have multiple locations.) So far, each time the items have been sold within the 48 time slot.
Sales have jumped, foot traffic has increased (they actually ask where you heard about them and WRITE DOWN where the customer tells them (amazing how many ask and then just say, "Huh... thanks.")
So there's definitely ways to leverage the service!