The stock alarm will only save you from a couple of things. First someone driving off with your car since the key uses electronics when starting up the car, you cant hot wire it. Second your stereo and nav will be useless to the theif if stolen because it wont work anywhere else, and not in a different G. But the alarm wont save your seats, rims, engine parts, anything else... Because all the person has to do is pop open a door (alarm goes off) open the hood and start pulling out the fuses that correspond with the horn and lights and your car is a sitting duck. But again totally depeneds on where you live, and where you park you car, and the places you go out at night where you might leave it then.
2003 G35C / 6mt / DG & Willow Leather / Nav / Premium / Aero / ETA December