Having been playing and reading Ridewithgps it suggests
- GPX Track (.gpx) Useful for any GPS unit. Contains no cuesheet entries, only track information (breadcrum trail). Will provide turn by turn directions (true navigation) on the Edge 605 and 705, but will not have any custom cues. Edge 605/705 users follow these instructions.
- Garmin Training Center (.tcx) Best format for turn by turn directions on Edge 500. Great for training when we release those features. Note: not currently optimal for Virtual Partner. Better support is being developed.
So I'll give this a punt