I love seeing examples of how Twitter has increased traffic, grown business, etc. I just started following
http://twitter.com/HowToMakeMyBlog (Marko) and he really has some good stuff. You should follow him. Here's a blog post where he shows his traffic results from Twitter.
http://www.howtomakemyblog.com/ebook...keting-e-book/
(I'm not endorsing the book because I haven't bought it but I just want you to see his Twitter traffic stats)
Shows you that if you learn how to work it for your niche, you can bring some great results. I am also starting to see more traffic from Twitter. I don't promote a lot of affiliate links on Twitter so I doubt it's done much from a monetary sense, but it is helping with traffic.
I have definitely learned over time what kinds of tweets people respond to and what get people's interest. And that's something I think everyone has to figure out for their own niche. The more I use it, the more I love it.