As someone who has worked with young students, high school teachers, university students, and university faculty getting Pi's into the classroom (e.g. http://clean.energyscience.ca/...
I can say that the micro:bit may be a better starting point for really young kids and their tech phobic teachers than the Pi. From what I can tell, the micro:bit isn't really a computer (unlike the RaspberryPi), but rather a peripheral that enables some physical computing.
There are some ugly sides to the Pi for the uninitiated. I'm not saying one is better than the other (I really like the Pi), but I do think the micro:bit could be a welcome addition to the ecosystem.
I'm disappointed that BBC isn't making them available to the general community from the get go (or even before release to schools). We have a way better chance and troubleshooting (and populating stackoverflow) issues than they do. Despite the fact that this is intended to be plug-and-play, things never are (especially when they involve locked-down machines like those present at most schools).
In any case, I'm looking forward to getting one of these things!