I get 14.7 mpg with mostly semi-aggressive city driving (shift at over 5K rpm on a manual transmission). I can get 26 mpg on the highway during long trips.
Since you are accustomed to the cost of maintaining a German car there should be no major surprises if you buy a 6er. Parts are expensive but the E63/E64 are fairly reliable. If you are worried about maintenance costs buy a car that has a CPO warranty.
By far the largest potential expense is the coolant pipe issue that can occur on any BMW V8 at the 75K to 100K mile range. It's a $1K fix if you DIY it using the All German Auto expanding pipe, or $2K for an indie to install. The dealer will charge at least $9K since their repair procedure involves removing the entire engine.
A few problem areas:
- Battery/alternator - lots of electrical systems requires a battery and alternator in perfect condition or the car will generate random errors
- You need to hook up a battery tender on weekends if you take only short (15 min) trips during the week
- Leaking timing cover gaskets - there are 3 and one requires the engine be removed to change
- Valve cover gasket - all BMWs will leak oil from this cover eventually - when the V8s leak you need to change 2 gaskets - lots of labour involved
- Plastic transmission oil pan on auto tranny cars - eventually fails
Maintenance is no different than your average BMW. My M3 cost more to maintain than my 6er. I do my own oil changes for ~ $100 (9L of synthetic plus a filter). I use hi-perf aftermarket brake pads at ~ $200 for all 4 wheels and OEM rotors at $150 each. You can go with less aggressive but still quality Centric pads ($140) and Centric rotors ($80 each). If you drive hard you will go though rear tires quickly - 360HP + 4,000 lbs.
Despite it's size the 6 series seems to defy physics and handles very well at the limit. I've taken mine to the track a few times and it was a blast. Make sure you get a car with Dynamic Drive (active roll stabilization), the system works wonders.