Just about any camera is better than no camera, but it is also possible you buy too cheap and luck out as when something finally happen you find out the camera stopped recording months ago.
We had a few come in here and tell that story.
In general i feel one camera on the windscreen is just fine, it might not capture all that go around your car, but it dont need to, it will log 100% what you are doing with your car.
And if you dont do anything stupid then what ever happen on or off camera must be blamed on the other part.
If you have the microphone on it will pick up your indicator, so on can claim you did not indicate. Also the camera will record you hold your lane so it was not your fault that idiot side swiped you as he claim.
So there is a lot of good in just one camera, and if you like me like to expose the dumbasses out there on youtube you can always expand your setup down the road.
I have more cameras in my car than a well equipped CNN studio, but by far most of the interesting things happen in front of my car.
So
1. dont buy too cheap
2. have a look at the footage on the SD card once in a while to make sure files correspond with the drivings you done lately, and have a look not least on last file in drive sessions to make sure it can launch in what player you use on the computer.
3. and dont go cheap on the SD card either, those little suckers are the weakest link, at least in good cameras.
And remember if you start small, and get hooked and want to expand down the road, you can always hand over the old camera to a worthwhile person.
My old Lukas is in my little sisters car, and i keep my friend hooked up too with cameras i received as free test samples after i have stopped beating on them.
And giving is always much better than getting, instant 1 up on the karma account