Just wanted to add a comment as this discussion was why I looked up this website before buying a lexus hybrid, the comments did prompt me to really think about why I was looking at a RX450 and my expectations for the car. I bought a new RX450 60 plate and am now averaging 37mpg, have had a best of 39mpg, this is mainly rural and A road driving, with some town. One long trip Motorways averaged 35mpg (70 - 80mph on M40, M25, M20). It seems to be becomming second nature now to drive more economically, I still put my foot down though, otherwise I know I would be getting better figures.
Only had the car a month but so far really impressed, was expecting 31/32 after reading the posts here and speaking to one dealer, but after speaking to others was hoping for 35/36.