I went to a Lexus open day thing up at Slayley Hall and got to drive a few of the Lexi(sp?). I had a drive of the GS450h but ran out of time to drive the RX. If boot space is a concern to you then the RX might be better as the batteries are stored under the floor on that, rather than taking up half the boot like on the GS.
When pulling away out of the hotel carpark it was silent, until we got out of the grounds and gave it a few beans. Then the engine kicked in, giving it decent performance and it did actually sound pretty good IMO even though it was CVT (although we didn't do it for long periods, it might sound a bit pap after a while!). Economy wise, these cars are supposed to be better at knocking around town than on a long run as the engine is always on at motorway speeds. The economy of the GS that we drove was appauling according to the OBC but that will be due to everyone jumping in and trying out the power down the country roads. I agree with the above that the dash isn't up to the standard of the German stuff, little things like the clock that looks like its been pulling from a 90s Toyota put me off. The screen which showed where the power was coming (engine/motor/recharging) from was very cool, but distracting!