I too have a clifford alarm installed with all the gadgets mentioned by blink and accented installed i.e. dual proximity sensor, windows sensor and smart windows, central locking sensor with automatic lockup over a certain number of revs, auto light sensor, headlight sensor, digital tilt sensor, glass break sensor, wireless immobiliser, audio sensor so that it does not false alarm when an asshole passes by with full BASE, omnisensor, ultrasonic sensor, and anticarjacking. I left the car for only one day and as blink said the installation is done really well as well as explanations and help. The cons are simply that it is quite expensive but my opinion is better safe than sorry, and before spending so much money in modding and sound, I advise investing in a good alarm.
Clifford also have gsm pager and intellistart if wanted but as accented rightly said pagers are thoroughly useless and dangerous for yourself apart from being ineffective if the thief approaches the car with a reception killing magnetic device that will easily block ur two way system or if reception is obstructed by buildings or if parked underground so its not worth the hassle. Better have a good alarm without two way IMO.