I've had my RX400H (an '07 SEL) for 14 months now. At first it felt very odd to drive, compared with my previous car, a Mercedes Coupe, but once I'd got used to sitting up very high and remembering to look down to see other cars instead of up, it was fine.
I now really like it. In fact, when it was serviced the dealer loaned me an IS300, and it felt small, cramped and even a bit slow(!) by comparison.
My RX has been absolutely no trouble at all, and over the year it's averaged 30.5mpg. It definitely does better in 'moderate congestion' - e.g. 25mph trundles round the M25 will often produce 40+mpg, but 75-80 mph motorway stuff produces 28-30mpg.
Overall I'm very glad I bought it!