Through this promotion you’re essentially paying 1.87 cents per mile, including tax. The deal used to be better, as the normal cost of miles was 2.75 cents per mile, before raising the cost to 3.5 cents per mile.
Another restriction this year is that they’ve limited the bonus to the first 40,000 miles purchased, meaning you can buy a total of 80,000 miles at the discounted price through this promotion. In the past the bonus applied to the first 50,000 miles purchased, meaning you could buy 100,000 miles at a cheap cost, which is enough miles for a trip to Europe, North Asia, etc. Obviously this is an attempt by US Airways to sell you miles as much as possible without you actually redeeming them, given that they probably expect people will buy the remaining miles for 3.5 cents each, plus tax.
As always, Dividend Miles accounts have to be at least 12 days old to take advantage of this promotion.
While the promotion is still a good deal, it’s not quite a “steal” anymore. I’d say take advantage of it if you have a specific use in mind and the numbers work out, though otherwise I probably wouldn’t just speculatively buy miles through this promotion anymore.