You have to watch out that you don't get the cheap cheap copy of the copy, otherwise the reviews are good.
The SJ4000 was very popular, as its been around for a while now, its one of the cheapest budget cameras, however I would strongly recommend the Git 1 or 2 instead.
I had a SJ5000+ from bangood.com, bangood support was appaling, I wouldnt buy anything ore than a few pound from them. The build quality of recent SJ cams isnt very good.
The Git 1 and 2 are excellent build quality and the support from the manufacturers is amazing, they listen to users and add new features in firmware updates, whereas SJCam just keep releasing new models rather than improving existing ones.
It depends what you want the camera for, and whether you just want the cheapest.
Git 1 and SJ4000 are 2mp, SJ5000+ and Git2 are 16mp
so if you want still photos, go for the higher spec. If you want the best possible lowlight video, go for Git1, the less pixels, the better each one is at collecting the available light.
I havent used Sj4000, and there are lots of good reviews, if you are going to get one, Magickelly got us a discount from gearbest in the above link.
most chinese supplied cameras only have a 3 month warranty be warned.
If you want a good all round camera, I cant recommend the Git2 enough, its a good car dash cam, you can but it with a standard wide angle 170 lens, or a narrow angle 72 degree lens, but these ones dont fit a waterproof case.
anyone fell free to PM questions re the cams...
Al