US Airways is running a 100% bonus on purchased miles again, up to 50,000 free miles. If you want to fly around the world in business class, 90,000 miles will cost $1,575. I wouldn't buy miles to hold onto them, but only buy them to top off my account for an award I'd want to book soon. These promotions happen every month or two.
You Don't Have to Fly US Airways When Burning Their Miles