Yeah, they completely screw up the car.
We had a lady in with a Z4 with an after market alarm system...and the managers employees (Circuit City) didn't install it right. After awhile the passanger door wouldn't lock/unlock....had to un-install it and then re-program the whole car...after a couple hours....back to new...WITHOUT the after market alarm system....
So...moral of the story....don't get one!
Unless you want nothing but trouble.