Here's a nice blog post on DOSHDOSH (http://www.doshdosh.com/twitter-marketing-mass-follow-users/) discussing the differences between the Quantity and the Quality of your Twitter followers.  The key point is that having followers that are responsive to your tweets is much better than just having a lot of followers. 

I have a couple of simple rules for Twitter:

1. Only follow people I have a genuine interest in.
   
2. Only tweet stuff I'd be willing to read myself.