I've taught them on and off. From the instructors perspective (at least this one), I don't care for them. No personal contact, threaded discussions aren't as lively as in class, and it seems a lot harder for students to get as motivated and manage their time when they don't have a class at a scheduled time. Still, some students seem to like them, and I think the applicability of them varies greatly depending on the discipline. Probably more appropriate in say business and accounting than say geography (more visual), but just my take.