Well, I just found this post thanks to a tweet by The Boss so ... that proves something, doesn't it?
Twitter search is useful to watch how your brand gets tweeted and re-tweeted, whether the tone is generally SiteXYZ rocks/sucks ... but you don't need to be a member to use search.
I'm not sure how useful twitter is, but subjectively, the experience is completely different from, yet somehow a blend of, blogging, instant messaging, and email. And the "community" (if it can be called that) seems a lot more promiscuous than, for example, linkedin or even facebook - people seem to follow and unfollow others liberally. So it feels very informal, and very easy to meet new people simply be following them.