Twitter can be useful for generating leads, especially if you view it as a way to respond to peoples needs than as an advertising tool.
For instance if you sell say..... t-shirts with ponies on. Then you can use a tool like
Tweetdeck to set up a filter so that if anyone uses the phrase "tshirts" and the phrase "ponies" in a tweet then if shows up in Tweetdeck.
Some of those tweets will probably be people saying "Where can I buy a great tshirt with a pony on?". If you reply to them and say "Come and look at our site we have loads of pony tshirts, plus if you use this coupon you get 10% off." then you probably have yourself a sale.
Twitter allows you to listen to what people want *right now* and respond straight away with a solution.