I'm only 5'8" and I thought the Continental GT interior was cramped up. Hell, if you go find the Top Gear episode, you'll see that Jeremy Clarkson seems to be uncomfortable driving it.
As for why some people would drive a Continental GT over an AM or Maserati, it's either merely a matter of choice or just a fashion statement. Perhaps their neighbor or friend already owns a DBS, so they'll try something different. Or maybe they drive one just because several football players own one.
The problem with the Continental GT is exactly the same as with the new Mini Cooper: it's a car sold only by its badge. Sure, it handles well and goes fast, but it is not really a Bentley other than having the badge on the front end. Though it is assembled in England, majority of its parts come from Volkswagen - especially the chassis platform and W12 engine, which are shared with the Volkswagen Phaeton.
At least the other models (Brooklands, Azure, Mulsanne) are genuinely Bentley, with their legendary 6.75 liter turbo Rolls Royce V8.