After having gone both through the experience of students commenting on each other's blogs and commenting on another class, the latter wins hands down. I now wish I had videotaped how serious the students were when they were responding to Ximena's class. The set up I gave them (this is an ENA 99 class, they need your feedback) make them see it as a real contribution and not a jump through hoops. On the other hand while there is a good chunk of people who take commenting on their classmates seriously as well, there seem to not do it with the same urgency.
Since we just had an English department meeting where we were discussing the value of classes of students with a range of abilities, I am starting to think that this may be where networked composition classes can really benefit students.