Immobilizer (theft control unit) replacement question

[kiyakubuke]

Ok, so has anyone ever replaced a bad Immobilizer unit (theft control unit). I am getting conflicting account on whether or not a used one can be used. some say it should just plug in and be fine (paired to the key) other say it has to be programmed to the car and that once programmed to one car you can't re-use it.
I have noticed that most of the used ones on eBay come with the ECU, keys and door, ignition and even glove box locks. and even they state that it may have to be professionally programmed.
insurance is paying for it to be replace, but I would rather do it myself and save the $500 deductible. if I have to pay $600-$700 for the part, and another several hundred to have the dealer code it. I may be out more than that.