I've managed to borrow a Garmin VIRB and I'm having a play with it.

Here I am on the tail of a couple of other riders on the descent into the beach town of Shek O on the south side of Hong Kong - quite a popular bike route.

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There are many pre-set formats for the data displays.

Good:
- quality of image
- sturdy mount, yet quick to unclip
- built in GPS and ANT+

Annoying:
- didn't pick up my cadence or power (only HR)
- has proven difficult to sync up my full data from my Edge500 (can be done easily, but it is not running in time with the images)

Still a work in progress. There must be a trick to matching up the data and video accurately.

Other thoughts:
- Helmet mount might sit quite tall. My Contour mount is very flat and the camera sits low.
- The free VIRB Edit software could be used to sync footage from any camera to data from any source (haven't tried, but presume so from the import options)
- lens cover is very domed, perhaps more likely to be scratched in use compared to the more recessed glass on other cameras.