I've had my IS250 since last July. In that time I've done exactly 5,000 miles (to the exact mile!) at an average 29.79mpg (calculated from actual petrol used, not from the car's computer). My best tank was 32.43mpg and my worst was 26.37mpg.
Most of the mileage has probably been motorway (weekly trip from Hampshire to West Midlands) with the rest around town. I've mostly used either Shell V-Power Nitro+ or Shell Regular Unleaded petrol in it, which has worked out (with petrol prices falling) at 19.31p per mile overall.