 I've owned a lot of Bimmers.  The E60 5-series 550i is my absolute favorite.  Very composed and agile for it's size, it comes with a screaming 4.8l V8 good for 360HP (or about 500HP if you put an ESS supercharger on it like I did).  I have owned mine for three or so years now, and the only issue I have had is with an intermittent clock reset due to a failing comfort access door handle.  It is roomy, smooth, and comfortable for those long highway drives, but one must constantly be aware that the car tends to want to cruise at it's comfort zone of around 85MPH - i have had many occasions where I am doing the 65MPH limit, get distracted talking or listening to music, and then find myself at close to 90 when I look at the speedo again.  The car is near silent and rock solid, so you really don't get the normal cues that you are speeding.  Gas mileage is the Achilles heel of this car, though.  I averaged about 15-16MPG in normal combined driving, which would drop to 10-12 if driven hard.