Be sure to ask questions when you go to buy.
The main questions are:
1) What is the Resolution and Frame Rate per camera. (Minimum 4 CIF and 30 fps / Camera)
2) Storage: How Big, How Many Days, etc.
3) Be sure to demo the system, look at the interface, ease of use, etc.
A lot of the cheap systems false advertise and do not disclose the actual Resolution.
4 CIF is not equal to CIF.. just look at my example, major size difference... also don't mess with Quads they are 1 CIF per camera... get something that runs each camera at 4 CIF or you will be hating life if something goes down and the image is so pixelated or small you can not make out who the person is, etc.
If they got a demo, they usually run it on the highest setting for resolution, that takes a ton of storage and retention is at its lowest.. so watch out for that if you demo equipment ask about the storage and days of retention on the highest and lowest settings and be sure the resolution is sufficient.
If it sounds to good to be true... it mostly is...