My IS is thirstier than any car I've ever owned including a 2.5 litre 3 series BMW, 2.0L Honda Accord, 2.0L Primera EGTE, and to top it off I think it's thirstier than my dad's old Nissan 300ZX twin turbo pushing 350BHP!
Driving steady in the 300ZX I could average 26-28mpg. Foot down was about 5mpg to be fair tho :)