FWIW ...
I recently decided that I wanted a dash cam as well. Started by checking the Which (UK magazine) article and then youtube videos for the magazine's "best buy" - ended up watching a large set of Russian dash cam footage (some absolutely incredible scenes there). Anyway, the result was that I bought a Nextbase 402G from Amazon and have been testing it out for the last 8 days. The results are extremely disappointing !
Generally, it's a great picture but sometimes mine shows a matrix of flickering white spots as if the scene was shot through a wire mesh then the dark image of the mesh was removed from the picture. Yesterday, I discovered that these are far less noticeable when viewing the movie through Quicktime under OSX instead of through the Nextbase video viewer ...
As Nigel said, a key thing is that you can read number plates on the image. With mine, you have to be no more than a car's length from the vehicle in front before you can read it. Not because of poor image quality, but because on the model I chose the lens has too much of a wide angle view resulting in the recorded images of vehicles being so much farther away than they were in real life.
Finally, there's an intermittent crackling / scratching noise on the soundtrack sometimes, and the camera's self levelling function means that "level" has around 3 degrees of list to starboard, and, in the recording mode I'm currently using, the picture keeps showing brief flashes of parts of the image with different exposures or changes of colour ...
Needless to say, this one's going back to Amazon (one of the reasons I bought there instead of in a shop). I'm tempted to replace it with a 302G, which IIRC has a slightly longer focal length lens, but I can't check this because Nextbase have "updated" their website and technical details like that are no longer present. Having just had a quick look at Dashcamtalk, it's annoying that they only seem to list American models and the Nextbase range seem to be absent - I couldn't see any there that looked the same even if they had a different name.
By the by, I almost suggested that Nigel was using a Nextbase cam because the images he posted look so much like the ones I get from the piece of junk that I currently use.
ATB,
Ro
Have to say that I was horrified when I found that the voice recording was on as I have to admit I hate to hear the recorded sound of my own voice...Just sounds so different, if you know what I mean!! 
