My 400h has been averaging 29mpg since I bought it three months ago (350 miles is the most I have got from a tank before I decided I need to fill up!). Must admit, I have a very light foot and I expected a lot better than this, but since most of the driving is local journeys I guess it's acceptable, and it's a hell of a lot better than my previous car - Range Rover 4.4 V8 and 15 mpg!
But I never bought the RX for it's economy anyway, it's just such a great car and for a 4x4 has pretty good running costs too. ;)