I have found that you need to grade the blogs, in order to get everyone in class blogging. Generally, I have about a 1/4 of the students who, once they start blogging, "get it."
One strategy is to just grade on whether they post or not, to get them started. If you randomly sample every week, and read or discuss posts from a couple of students, that kind of gets them working.
A good collaboration is a joy and usually leads to jobs and money. That said, not all collaborations are "good." Lots of students hate working together, so the group blog is tricky, isn't it. I still believe that they need to learn to collaborate, I'm not just sure of the best educational strategy to encourage this.
I've done individual blogs and class blogs. They serve different purposes but I think individual blogs are particularly good for helping students develop a professional online identify. You can see my students' blogs on San Francisco neighborhoods at Find out more at http://ieimedia.com.
I give one grade for the semester-long blog. I have a grading rubric that gives points for writing; reporting; use of the medium; creativity; usefulness; grammar/style/punctuation.