I agree with everything you said, Filthy Monkey. In fact, I'll add a couple of things that I'd experienced buying an M6 when I already had an M5 ...
On paper my feelings about the M6 were:
1) I am actually giving up several much-loved features that the M5 offered (sun shades, extra passenger room, more spaciousness, moonroof!!, folding rear seats, comfort seats, seat ventilation) and a luxury car feel when not using P500.
2) The M5 actually does slightly better in the handling department according to many track tests.
3) If I'm spending $110k on a car, why not just get a porsche?
4) I like many aspects of the M5 styling better.
... and then I drove the M6. Believe it or not, it does seem a lot different behind the wheel than the M5 and, like many say, sharper and more focused. It also turns way more heads when I'm driving down the street (people drive up next to me and make a very big deal even though it doesn't really stand out much because it's black/black -- hard to tell apart from a 650) not that the extra attention makes me like the m6 more than the m5, but to all those naysaying bangle-busters out there, these cars do get huge amounts of positive attention from the general public!
I'm still below 1,200 miles, so I don't know what it's going to be like with P500 on, but the car already seems more responsive to the accelerator and nicer in the turns so I'm very optimistic.
The M5 is more comfortable, and is better to cruise on the highway with for sure, but the M6 with Comfort Mode on is a lot smoother in San Francisco than the M5 was, and the M6 somehow suits my state of mind better. I'm also finding it more fun to drive. The cockpit seems a bit tight after driving the M5 so much. I'll let you know in 600 miles. I have a Black/Black 07 M5 on order on standby just in case.
And to answer (3) Why don't I just get a Porsche ... well the only model I really want is the GT3 and I'm having a hard time getting my hands on one. Besides that the roads in San Francisco suck! making the GT3 a horrible thing to drive, and Porsche doesn't offer 1/10th the warranty/maintenance/service/support that BMW does. Try paying for your oil changes again and having your car in the shop to get tuned and see how much you like them. Maybe the 997 turbos are different but my friend's 996 GT3 was in the shop often enough to piss him off ... and he had to pay for it every time.