I don't use this all that much, it's mostly my hubby's mid-life-crisis-mobile! Poor love, but still he does love it so. I have driven the M6 a few times, and been a passenger a lot more than that, so I'm fairly familiar with it.
The first warning is the sheer cost of running it. Really the fuel consumption is just silly. We often get less than 15 miles to the gallon, which means regular stops to fill up, at such high prices these days. Another warning is that the back seats are a bit of a silly size. Our little ones are not so little any more so that isn't much of a concern for us. We're more likely to be transporting grandchildren around, which the tiny back seats are actually just fine for - not to mention the kids really love driving in it! I suppose that's one good thing, makes them want to spend time with their old grandparents! The BMW M6 is strictly two people only in the back as well, since there's a rather arrogant central divide that sits where a central seat would be.
The seats I have to say are very comfortable. We've had a few longer trips on it, although mostly we use our Honda for those as it's cheaper on petrol. The only problem I would say is that the seats are rather low to the floor, which makes my knees a little tired after a while. That's no doubt just my advanced years though!
There aren't a huge number of spaces to put things inside the car. One thing that has tested my patience a little over the years is that there isn't a pocket by the door handle. In most cars there's a little space just next to the door handle where you can keep just a comb and a pen, that sort of thing, but on the M6 there's only the big lower door pocket, which you can't see to the bottom of, so it's tricky to have little things handy inside the car.
Look at me, I haven't mentioned anything about what it's like to drive! Well suffice it to say, the BMW M6 is very fast indeed. My only complaint would be the 'SMG' gearbox thingy, which took me a while to get used to, and I still find a little bit unpredictable. I don't think that's just me being an old luddite, I am a fairly experienced driver and it just seems like it's not quite finished. But hubby just says that the whole point is to get to top gear and stay there! Ho hum...