http://commandoalarms.com/itm30031.htm

The other day an idea came to me and I figured somebody was already selling it. There's a lot of complicated/expensive car systems but I was thinking a sensor that would alert me if the car door was opened would be all that I need, like a keychain that vibrates if the door is opened.

I'm assuming that's basically what this is. Has anyone had any experience with this device or know of something similar that has worked well? Are there any downsides? What about lojack and other more expensive solutions. Are they worth the price? Or does the insurance discount balance out the extra cost?