The BMW alarm is as good or as bad as any other, maybe better than an alternative in as much as it's integral rather than being lashed on.
Will it stop your bike being stolen? Lots of bods will report that their bike has one fitted and it's never been pinched. They also have a pound of butter in the fridge, which is just as likely to have been the deterrent.
The only way to do it best? Chain your bike to something solid and buy decent insurance. Don't rely on a disk lock or a chain through the wheel, left dangling or lying on the pavement.... Or an alarm, alone.