How I Use It
I use it primarily to send out reminders about due dates, pose questions to extend in-class discussion (or prepare for it), and field after-school questions that students have. It's great because students can quickly tweet a question, which they do more readily than email. It's also awesome for extending discussions from the (often-too-short) day's class; sometimes I toss extra credit to students who respond. Overall, my students have responded positively to it and it allows class to be fluid instead of timed by bells.
I set up a Twitter for my classes at the beginning of the year, and encouraged students to "follow" it. Before setting it up, I was concerned about maintaining boundaries, going about social networking in a professional manner, etc., so I researched how other teachers were using it.
Twitter allows a level of higher interaction without crossing the line; I never follow students back, which keeps a level of professionalism - I am their teacher, not their friend. The downside is that not all students choose to follow (or use) Twitter, so the messages only reach those who choose to engage. At the high school level, though, that's reasonable.