I've used the Eagle Tree Systems datalogger quite a lot for "development" work, it's a small light weight device that records in-flight data. The thing is easy to install and remove. I've used it to measure altitude, airspeed, G-forces and the like to choose props, evaluate maneuver geometry, evaluate airframe loads, etc. The information obtained is quite useful as it shows what really is going on in the air (which is not always what conventional wisdom predicts).

For E-power I would add the current sensor (0-100 amps), simply a small sensor that slips over one of the power leads, a temp sensor, and a motor rpm sensor. While the voltage is also worth recording, the system requires a common ground (with the rec battery which powers the unit), this might not be a good idea - need to check further. These data would be useful in ensuring that the in-flight current / temps are reasonable for a basic set-up, as well as an aid in choosing props, etc.