I'm often asked if it's worth it to buy frequent flyer miles from the airlines, especially when they have bonus opportunities. As with many questions, the answer is, "it depends."
American Airlines is offering a 50% bonus opportunity until the end of February. If you buy 60,000 AAdvantage miles, you get an additional 30,000-mile bonus. The cost? $1650. (US Airways has frequent 100% bonus mile offers, if that's your airline of choice).
Worth it? Depends on how you spend the miles. Let's say you were contemplating a business class trip from New York JFK to Tokyo Haneda on American's nonstop, traveling outbound March 4, returning on March 13, 2013. A business/first fare on American would cost $5136 round-trip.
But with your $1650 mileage purchase, you get almost all of the 100,000 AAdvantage miles you'd need to obtain that ticket, after paying $42 in taxes (plus, you'll earn 3200 miles with your Citibank AAdvantage card).
So yes, I'd say it's worth buying the miles in such a scenario (I'm assuming you already have 10,000 miles in your account).
I also looked at business class flights from LA to Honolulu, which you can get for 75,000 miles round-trip. The fare would cost at least $2348 currently. So again, you'd come out ahead buying the miles, and have 15,000 miles left over to upgrade, say, a $159 one-way flight between LAX and JFK to a business class seat that might cost $2000 or more (something I do frequently).
Short answer, yes, if you fly in business or first class, buy the miles. However, if you do most of your travel in economy class, then skip this sort of offer. It's just not worth it.