Alarms are only any good if they people pay attention to them when they go off and most don't. I like the idea of the siren in the car, but with the way things are going today, the little scrote who steals the car would probably sue you for hearing damage!!!!
Most car theft is by opportunistic little shits who only steal unalarmed cars, however, there are lots of organised theives who don't care what alarm you have since they usually know just how to either bypass them, cut them out quickly, or disarm them using factory overrides.
There are lots of tracking devices around, but again a good car thief will know where they are located, and how to strip them out.
This device sounds good since it can be placed anywhere you the owner want to put it, and it doesn't have to be hard wired, making it impossible to damage it with a voltage spike to the electrics.
When my car gets to be nearly complete, I'll be having a toad alarm fitted somewhere very inaccessible, with sirens in the car and a tracking device of this style also.
If you've spent the time and money on your car, you want to make sure that it is either no stolen, or if it is, you get it back in one piece.