Tracking systems

Does anyone have a view on the best trackers for caravans,I'm after a system if my van is stolen i get notified or i can log on to a web site at any time and get the positioning of the van,but there seems to be so many companies offering there services and all at different costs for purchase and airtime.

For me word of mouth is good and id like to hear from people who have them fitted not just what the website or sales person has to say.