Overall, I really liked using Blogger. It was a little time consuming to set up initially (though not as bad as I’ll make it sound) but once we got going, it worked great. I’d say that total process of setting up 16 blogs took about an hour of my time, and then about 10 minutes per group in class. The most time consuming part was creating each blog. I also spent a little time designing a template, though I had some experience from my site. I just used one of the standard Blogger Templates, then took out the navbar (and Next Blog Button!!! directions are here) and some of the other sidebar content. I then added a blogroll to link all the blogs together using Bloglines. (The template is a kind of weird hybrid of CSS, html, and some unique Blogger things. The Blogger help section has a good walkthrough).
I retained the master login for each blog. This allowed me to edit and delete posts at will (though I only had to do this one or two times with 100 students). Once the blogs are created, you can invite others to join through their e-mail. You have the option to give the individuals who join full editorial privileges, or only the ability to post and edit/delete their own posts (I chose the latter). Each student then received an e-mail inviting them to join. They each got their own unique login and password. (When I did the Responsible Blogging Lesson, one of the very few rules I myself added was that individuals were responsible for what was done on their login, so if they shared it with a friend that was at their own risk)
I hope that answers your questions. Please let me know if you have any more.