If you read my recent review of the Boeeerb MotorPiTX, you will be aware that I had trouble with powering the Raspberry Pi and the MotorPiTX board from the MotorPiTX power socket, and resorted to ignoring the smart power circuitry and using the usual Raspberry Pi micro-USB power input. I'm happy to report that this was not actually a problem with the MotorPiTX board, but rather with the way I had chosen to power it.
I had been doing some experimentation with battery-powered circuits, particularly using an Arduino Uno, and I had on my desk a 9V PP3 battery pack with an appropriate barrel jack, so it seemed an obvious choice to plug that in to the MotorPiTX power input. The 9V was seemingly enough to power up the MotorPiTX regulator circuit, but I now realise that the battery was not pushing out enough juice to reliably start the Raspberry Pi. This was particularly apparent because, althoiugh the lights were flickering, it never got as far as displaying anything on a monitor, or even attempting to connect to the network. When I replaced the battery pack with a 9V 500mA "wall wart" power supply the Raspberry Pi started with no problems. The Relay does still clatter a little when power is first applied, but after that, the little button works great.
It seems that the system, when powered from the MotorPiTX board, is pretty sensitive to power, though. Flushed with success after getting the Raspberry Pi started and stopped a few times, I tried to run the simple motor control example I used in the previous post. I'm guessing that this drew too much power from the little wall-wart, as it immediately removed power and crashed the Raspberry Pi. A bit discouraging, as the whole point of the smart power-supply circuitry is to help prevent such hard crashes. I'm sure that with a sufficiently powerful energy source this would not be a problem, but it is pretty drastic. Presumably this could happen at any time to a deployed project when a motor happens to ask a bit too much of the available power. In an ideal world the CPU would keep running and be able report an error or try some other fallback if a motor fails or draws too much power.
For a reliable system, I would either want some sort of "ring fenced" power available to the Raspberry Pi which can't be drained by motors, or (probably more likely) two separate power systems, one for motors and one for electronics. If we still want the comfort of a smart power supply, then a separate unit such as a Pi Supply seems a reasonable choice.