We're investigating it for my day job - but have not done much. I used it to promote a recent seminar, but had trouble figuring out how to track results. I've seen folks who just spam tweets out there and I usually immediately un-follow. For a retail shop it seems like a great way to broadcast occasional deals to your followers. But I'd say email marketing would be the main driver with Twitter/Facebook etc supporting those plans. my $0.02 anyway.
--Jack