I had the "worlds uggliest" softtop on my old convertible and that got knifed, they didn't bother trying to steal anything, just knifed it for the sake of it... :cry:
The best place to fit an alarm is inside the car, two or three extremely loud sirens inside the car and they won't stay in it long enough to steal anything.
I used to leave my 820 unlocked on the drive for all the reasons given above, but after the second time of somebody going in it and leaving the doors open for nothing, I've started locking it again.
Sometimes you get to the point where you wonder why you bother having anything nice, (since when was an 820 nice ?), just so some scum can come along and steal / wreck it at will, and there's nothing you can do about it......