We started using it sometime in February as a part of our Work on Writing station. The kids love it. Sometimes I have asked them to comment about something I have posted. During our poetry unit, I would ask them to write some poetry. A couple of weeks ago, we started a fairy tale unit, so right now they've been working on writing and typing their own "Once upon a time" three-little-pigs-style fairy tale.
If you are considering finding an online platform for students to write and respond to each other, I would definitely recommend Kidblog. Just like the homepage advertises, it's very safe and very simple to get started.