Started with Garmins about 7 years ago, and as they were stolen replaced with cheaper cameras (under $100) that had good reviews. (Tweakers steal them, either because they think they're worth something or they're just paranoid when they see a camera.)
They all seem pretty reliable, you just have to remember to blank and condition the microSD card periodically, and the cards eventually need to be replaced.
They help with gangs that commit insurance fraud, the average guy that tries to get a new car out of a minor bump, and can prove you weren't speeding.