Our University has seen an indirect benefit from it. We've recently implemented a Media tool to help us monitor and get a better handle on the various media outlets and thier benefits to us.
As for eCommerce use - I'm leaning more and more toward thinking that unless you're in a niche market and actively working the ropes (established yourself as respected member of that community), then you're just another noise in the dark or you may have a long way to go to establish yourself.
The few clients I've had that have used it have seen modest success but I won't go as far as to say it was soley because of Twitter. I think it was more about seeing my client and their products mentioned on multiple social media outlets.