Well i've found one fundamental design flaw which means i WONT be buying the Viper 791... Something Chris and other may have touched on, but not clearly explained. If i was to buy the viper, i would be buying it as a remote start and turbo timer TOY, not an alarm or immobiliser, not for security.
Immobilisers normally have circuits which are Normally Open (NO). This means the immobilisation circuit is open, and car will not start, until the alarm module disarms, and applies power to the circuit. Cut power to the alarm, and the car will not start, because the immobilier needs to power up the relay to close the immobilised circuit.
The viper is a Normally Closed (NC) system, the circuit is normally closed, and the alarm must apply power to break the circuit. ie, cut the one power wire to the alarm, and the immobiliser will not keep the circuit open, meaning the circuit is not immobilised and the car can be started. Not a good immobiliser!