Floni-
You gotta be *NUTS* to live in NYC and pay the insurance premiums here for theft insurance (which are a LOT) and not have something like Lojack.
The system costs $600 to install, and it's a one time fee. Think of it this way:
You spend $4k a year on insurance. Let's say $2500 of that is just your comprehensive coverage. If you save 15% on that = then you've saved $400 per year.
Inside of 2 years - you made your money back, and saved another $200. Totally worth it.
Can it be defeated? I've heard it can - and because I work in a security related field, can't discuss exactly how - but I can tell you that it has a backup battery, that is concealed within the unit - which itself is hidden on the car, at different locations within each one.
Honestly - it's a judgement call as to how effective it is. But I'm in it for the money

call your insurance company, see how much it'll save you - if you make the money back inside of 2 years, go for it. I have a friend with a Vette that pays $7500 a year in insurance. He was skeptical at first, because the dealer wanted $1k to install the system (some BS about it being a vette). Told him to go visit another dealership, who installed it for the standard $600. He saved almost that much in his first year of insurance alone.
Onstar - check it out
here. Not exactly what I have (TeleAid is MB's equivalent) but it's pretty cool. Not worth the money, IMHO, because of yearly fees. I'm not down with yearly fees
Good luck,
-J