i paid up for the extended warranty on my GS purchase as well. I normally dont buy these type of things but the Lexus one read fairly comprehensively and you are getting AA road side assistance thrown in which costs a few hundred quid anyway.
you have to treat it a bit like an insurance policy ... "in the event of" ... and going on the basis of Lexus reputation i would like to think that if anything major happens within the warranty period having this will be beneficial.
as i said i normally dont touch these things with a barge pole, and do not consider the Lexus to be something i would be worried about but my wife is a community nurse and her office is the car so with the AA cover etc this was useful
just my take on it ...