I've been mulling this over for a while myself, I've got an old (Very Old, Pre Go-Pro old) "Action Cam" which I've taken to popping on the bars, but it's not very good.
After looking at the footage from that and comparing it with other peoples youtube posted commuting footage I've decided I need a better forwards facing camera (probably with a wider viewing angle, most likely mounted on my noggin too) and something pointing backwards, possibly my current camera (but its a bit crap TBH)
It seems like capturing the before and after moments of a bad pass and having a better chance of actually getting the drivers face and front and rear plates and more of the manoeuvre helps build a case (should it come to that).
I've looked at a few cameras now, and Drift seem pretty good VFM Vs function and quality of footage, but paying for two of them does start to seem like a lot of money "Just" for commuting use.
Sifting the web and ebay there are cheaper alternatives in the £60-£100 range but it does feel like you'd be taking a bit of a flier as the claimed viewing angles/resolution/frame rates/etc can vary quite a bit, obviously it's whatever the far east are churning out this week (much the same as LED lights), while my instinct is to cheap out a bit on something liable to get bashed/dropped capturing decent footage of any near misses/incidents I might be involved in is quite a key issue...
Or has anyone found a decent cheap (Sub £80) cam?