Immobilise is a great system which works.
I was involved in some of the development of it into the bike sector a few years back – it was kick-started by mobile phone companies – and its biggest strength is that it is probably the only system used by nearly (might be all now) police forces in the country.
One police officer said to me recently that bike theft in the UK is staggering and there is no sign of it reducing. Amazingly most thefts happen from the home, ie, garage / shed etc. Systems like Immobilise are really useful in getting bikes back to their rightful owners.
However the main downside is that if the frame number is removed it makes identification even harder. This is why some companies are now trialling RFID systems to either replace or compliment frame numbers.