In reply to DEvans:
I was marshalling a TT on Sunday morning and one rider had a blowout just up the road from me and at that instant his head unit for his SRM power meter came off of its mount (presumably the shock?) and skidded across two lanes of dual carriageway before being munched by several cars. £500 to replace?
For me, that was reason enough to stick with a little Garmin Edge 500 and not put several hundred pounds worth of mobile phone in that vulnerable position.
I've also seen a Garmin 800 pop out of it's mount and do a similar thing on a club run. Again a biggish brick like thing on your bars