There seems to have been a big rise in popularity in these recently, so there are a lot available out there. The guy on YT that Pav mentioned is called Technomoan (if we're thinking of the same person!), he does some good reviews and comparisons etc.
As for whether they're worth it or not - You would hope that you would never have to rely on it because that would mean you've had an accident, or someones damaged your car...
The action cameras are a good choice because they do essentially the same thing as a dash cam and can be got for quite a good price. Just depends whether you want one permanently wired in, or one you can take out when you want.