Which dash cam?

I've been thinking about one for a while after several near misses by wreckless drivers, but there is a myriad of products on-line.

Ideally, I'd not want to spend more than £100, have decent HD image quality, and as small as possible.

What should I look out for when buying? I've been looking at Amazon reviews and nearly all of the products reviewed are like Marmite...hated and loved all the same. Every single one seems to be made in China (i.e. will fall apart very soon).

There isn't by any chance an iPhone app? One which overwrites every 30 minutes or so...? It'd work a treat in the background whilst having my phone stuck on the dash (using Sygic SatNav)... (for those budding App programmers, I'll post my account number later for the 10% royalties for this genius idea

Ideas?

Thanks