I have the Viper 5901 with back up battery (DEI 520t) and proximity sensor (DEI 508d). I LOVE IT!!!! Cant say enough how much peace of mind it gives me to know that if anyone gets near my Jeep I will know about it.
The back up battery is there in case someone cuts the battery cable my alarm will still go off. And the proxy sensor is like a motion detector, if someone gets close to the Jeep it beeps a few times to let them know I have an alarm, but if they reach inside then it will go off.
I HIGHLY recommend this setup, just remember if someone wants it they will get it no matter if you have an alarm or not. But the peace of mind is worth the cost.
Also everything you need to know about installing an alarm on your Jeep is on www.the12volt.com