I've had my IS250 since last July. In that time I've done 4,600 miles at an average 30.15mpg (calculated from actual petrol used, not from the car's computer). My best tank was 32.43mpg and my worst was 28.01mpg.
Most of the mileage has probably been motorway (weekly trip from Hampshire to West Midlands) with the rest around town. I've mostly only used Shell V-Power Nitro+ petrol in it, which has worked out at 20.01p per mile.
If you can find a MultiMedia version, the satnav is pretty good (it will dynamically reroute around traffic). You can use MP3 CDs in the Mark Levinson 6-CD changer, which allows somewhere between 80 and 120 songs per CD (depending on size/quality), so you can carry around 600 songs at a time. I also have a small DAB radio plugged into the AUX socket so that I can listen to Radio 6 Music.
The 2009MY post-facelift cars have a few nice, if not essential, features over the earlier cars (e.g. ability to autofold mirrors when locking the car).