Sorry to be a killjoy, bit it's dumb.
As is comparing it the the BBC Micro of old.
What made the BBC micro attractive was that it was a fully functional computer (of it's time) in it's own right, which you could start in a simple language drawing lines around the screen, making laser zap sounds, and have it prompt for the child's name, and then move on using the same system all the way through to structured programming, assembler, industrial control, simple office applications and data gathering and display. A cheap laptop with a suitable application is going to be far better.
This microbit is a toy that cannot exist without another computer being involved. It's going to be seen by the majority of kids as 'just another thing they plug into their PC with flashing lights' in the same vein as computer controlled toy missile launchers or bluetooth controlled RC cars. It will hold their attention for a few hours (if they get it working) and then be discarded.
I'm not saying that it will not get any traction. There are bright teachers (and some self-taught kids) who will do some tremendous things with it, I'm sure. Just don't expect this to be a significant part of teaching advanced computer interaction to the generation of kids who will receive it.
They'd do better going back to Logo controlled turtles, or a Kim-1 development kit.