Like anything else associated with the Internet, 95% of it is nonsense (uh, except for CB, of course, which is probably 99% pure gold).
But, some of my daughter's teachers tweet extra credit homework or to alert students to an article to read.
Many businesses tweet special offers to valued customers. I read of a small bakery that tweets "what's fresh" that morning, and saw their traffic increase by 10-15%. Businesses can send out tweets to valued customers to get instant feedback on their ideas.
Many hotels keep track of twitter feeds that mention them negatively and try to immediately reach out to the "tweeter" to solve the problem, increasing customer satisfaction.
The 2009 street protests in Iran were largely organized over Twitter:
http://www.time.com/t...,8599,1905125,00.html
This article is a pretty thoughtful writeup of what's good and bad about Twitter:
http://www.doshdosh.c...-you-can-use-twitter/