They are worth their weight in gold. I got one for each of our vehicles after a buddy was sent to the hospital from a head on. He had zero memory of the accident or how it happened. The other driver claimed it was my buddies fault, and actually came to the hospital to see my buddy. Guy got there as the police we taking the statements. My buddy told police to get dash cam out of truck, and other driver heard this. He actually called his son and told him to get to the wrecker yard and steal dash cam, but police beat him to it. Dash cam showed other driver, come across center line and hit my buddy head on. My buddy was in a s10 pickup, other guy was in an expedition. After the video was viewed by experts, showed that the other driver was looking down before impact. Medical record showed this guy had experienced multiple fainting spells and was doctor ordered to stop driving, but still had his license.
My buddy still has pain in his right leg and ankle (they were crushed) but at least he was not blamed for the accident as other driver tried to do.
As for brands, they are all very similar. Depending on your vehicle and age, a front and rear facing camera might be right (normally better for kids and young drivers) and I use forward facing with crash detection and mine turn on and off with the truck, but in busy parking lots, I back in and leave camera running.