I will see for myself when I sit in one in a few weeks time, but at just over 1.5" less headroom than the X5 I'm not worried about space in the X6 - it strikes me as being on a par with the 5-series (give or take a haircut).
As for the 5th seat. I can understand BMW's product strategy, but only time will tell if customers agree. Ultimately 'we' will strongly influence BMW's plans for the future if the lack of a 5th seat stops us buying the X6.
My view is that BMW should have allowed a baby-seat option (as does Volvo) to at least assist those people with younger families, but I don't believe there is sufficient width for 3 people in the rear whilst still keeping the heavily bolstered bucket seats (which I certainly like).
p.s. BMW might (reasonably) argue that even a Maybach only has 2 seats in the rear..