Bhutan is not for the impatient, the faint of heart, or the road-weary. Avoid it if you are any of the above. It takes hours to traverse the country on its one main thoroughfare and the endless twists and turns rival any roller-coaster you've ever encountered. You won't be able to squire yourself around there, anyhow. That's left to professional drivers. Should you find yourself in a taxi, never hesitate to ask him to slow down.
A series of small airports is underway to ferry travelers around Bhutan in 2011 across the country in less time; some day soon, Bhutanese who've never seen a plane much less ridden on one will be a thing of the past. Opening dates have not yet been scheduled at this writing.
For the repeat and super-adventurous traveler, it is possible to arrange to enter the country by vehicle at two points across the border with India, but that adds further complication and time to your journey. Ask your tour operator for details.