Twitter is its own community and platform; you shouldn't go into it just to promote your blog, because the LAST thing the world needs is another outlet for broadcast self-promotion.
Twitter is like anything else on the internet; what you get out of it depends on what you put into it.
It's the #3 referrer to my blog, but that's because I put time and effort into it. It gives me great story ideas, great blog material, has helped enormously to grow my community and raise my profile, but I've tweeted over 100,000 times.
I've gotten trips, meals, books, and friends out of twitter. But that is because I put a lot INTO Twitter.