Most cars that have had them fitted or already have them installed from factory are usually because they are uninsurable for theft without them and therefore have to have them fitted to comply with their own insurance companies requirements, eg:- evos, M cars, imprezas, ferraris etc.
I fit cobra trackers and TRACKER themselves where i work.
Yes they are a good idea because it covers u and helps if ur car is stolen.
It will bring ur insurance down but its wether u can justify the cost for one and if u actually feel u need one