As mentioned above, I made one, not a great success I have to say, but interesting to build. Brushed motors are much easier to use, as the controllers need to reverse on the fly very quickly to maintain balance. The vast majority of readily available BLDC controllers won't allow going from forward to reverse without stopping first, most will lock out if you try. My simple BLDC controller, with just a simple hardware "brain" would allow reversing on the fly, so two of them could perhaps be adapted to drive a balancing scooter.
The balancing control loop isn't too hard to design, although it needs a fairly fast processor to get the PID loop running fast enough and to process the signals from the accelerometer and rate gyro quickly. Mine was a bit "soft" feeling, so I suspect the loop wasn't really running fast enough.
The other point worth noting is that it's easier to hurt yourself with one of these things. The Segway has lots of safety features designed in, and uses very well tested code. A DIY balancing scooter will almost certainly be less safe and robust. When the code glitches, or something daft happens, like the batteries sagging because they're nearly flat, these things will just throw you off, usually with a violent face plant with hour hands too low to stop them from coming up and breaking your fall. Not a real problem as long as you know this may just happen fairly randomly, and as long as it doesn't happen at speed...........
The most useful site for DIY balancing scooter stuff is Trevor Blackwell's pages, here: http://tlb.org/scooter.html
