Depends on a few things. Mainly is do you live in a state that has "At fault" for collisions(PC term)?
If you do, then yeah, its a great idea to cover your own ass.
It also depends how deep you wanna go, cause you could get a rear camera as well.
I think they can be really good, and help your case out if you live in a state that has At-Fault driving collisions.
The state I live in MN does not have At-Fault. SO its rather pointless for me.