We have used Kidblog with our fourth graders and have been pleased with the ease of use.
i used Posterous with my second graders last year. i use it with my fourth grade this year.i like that my students simply use their school division email accounts to post and reply. it saved me the additional step of creating user accounts on Posterous. There was no need to remember user names or passwords. It helped that my division had Desk Now for every student as a feature of their network account.
Have you checked out the webinar by Linda Yollis? She did a live show on March 24 on Classroom 2.0 and I got a ton of great ideas from her presentation. She also shared a lot of resources and ideas on how to blog (which she does very well with her 3rd grade students) which I found to be very beneficial. Check out the archive on Classroom 2.0 using iTunes U to find it! Hope this helps!