It depends what you are looking for, My son has a B40/A118c "wedge" which is probably the neatest /best looking available with great picture quality, but is front only, these are about £50 on ebay , I wanted "quality" and something that would last, so went for the blackvue DR650 with front and rear camera's this was £239 from Ebay with a free power monitor, which switches off power if the voltage drops below 12v, so it will continue to monitor the car when parked(my car was recently keyed, so if it happens again, maybe I will record the scum)
Whilst the build quality and general feel of this is solid, I would not say it is as good picture quality wise as my sons cheaper unit.
Installation is relatively simple, the power feeds can be taken from the fuse box in the passenger footwell using piggy back feeds, or direct from the cigarette lighter if just using the plug, but then you would lose the parking mode.
The cables are simply tucked behind trim panels, the whole job shouldn't take more than an hour.