I drive mine on mainly country roads to work and back (a 24 mile round trip) and do overtake cars sometimes plus its used for knockin about after work, i usually get about 25 mpg but its so hard to keep calculating at petrol prices keep fluctuating.
when lookin at an IS bud check the belts bin replaced or take some torx screwdrivers n take the cam cover off and inspect yourself, check for front inner edge for tyre wear (some suffer more than others from what ive seen) inspect ball joints and just generally listen for any abnormal noises when driving it, it is a very relaxed and quiet ride so any abnormalities shouls appear, also check the cd changer loads & ejects cds and finally enjoy!
i love mine and so far i aint sin anythin out there that looks as good and drives as smooth as one of these.