I've only owned this car for a couple weeks, and I'll try and add updates to this if major things happen or change.
For the money it costs to buy one of these in decent condition, you can get a late 90s or early 00 model domestic car. And frankly if you can't deal with the feel of an old car, or give up some features of newer cars (such as super quiet engines, smooth idling, and minimal wind noise), then just get a newer domestic car and be happy.
If on the other hand, you can put up with a few shortcomings, and want a car with character that will make you smile every time you think about it, then get a 6 Series.
The engine does idle a bit rough (normal for M30 engines) and you can hear it chugging away at highway speeds. Also, unless you luck out and get a car with all perfect seals, there will be a little wind noise somewhere at highway speed. Mine has some wind noise coming from the sliding roof, but overall it is in pretty good condition for the mileage. When I was shopping around, I found a few other ones in the 200K mile range, and they were still nice as well. I understand the engine, if properly cared for (hard to tell on a used car), can last 300k+ miles.
The 6 series is a luxury car (80s luxury), but first a driver's car. The engine has more than enough torque and power to rocket you onto the freeway and merge between traffic. I've seen 0-60 times of 7-7.7s listed for this car, and cannot confirm that, but it's definitely fast enough for everyday driving. On the flip side of that power, the brakes are powerful, and even though I do not have ABS, the traction of the tires is enough to stop quickly and safely, even in slick conditions.
This car is most at home on the highway, although don't underestimate it because I say that. It can handle turns quickly, and let's you know when it's approaching the limit. I've swung out the rear end once on a wet road, and felt no need for panic, as it recovered itself quickly as soon as I let off the throttle.
The interior of this car is great too. I have standard leather seats, which are relatively comfy and can be adjusted 5 ways. The HVAC controls allow ample control over the airflow in the car. The visibility from the driver's seat is better than any car I've ever driven. All the windows are big, and looking behind, there are no large B or C pillars blocking the view.
I put the running costs at 5/10 for two reasons. First, I don't know how much it really costs, since I haven't owned it long enough to tell. Second, if you really want to enjoy the car, you will pay for gasoline. I tend to drive spirited, and typically shift around 2500-3000 RPM. Yes, 2500 RPM is spirited in this car. However my fuel economy has been about 15-17 MPG for city driving. Not too bad considering the return you get in sheer driving pleasure in my honest opinion. However, if you will have a hard time spending $40 a week on gas (@ $3.50/gal), then don't get this.