Aftermarket alarms

I've seen ads in the various BMW magazines that keep showing up in my mailbox for aftermarket alarm systems that tie into the E46's prewired setup. Has anyone used any of these and do they work as well as the dealer units? As it will soon be time to renew my auto insurance I'm wondering if an alarm will reduce my premium. Of course it depends on the insurer, but I thought the factory/dealer price a bit steep. And since we have that silly police flasher under the rear mirror I thought I might actually use it. Any comments or experience would be appreciated.