Normally 3 Amex points transfer to a single Starpoint.
With this bonus the ratio is 2:1. That’s not necessarily a good deal. Membership Rewards points, by a wide margin, are best used (1) transferring to airline miles (2) during a bonus promotion (3) for low-level premium cabin award redemptions.
Hotel redemption are generally not even close in terms of value. In most cases, even with this promotion, they’re still not. It takes 24,000 – 32,000 Amex points for a single night’s stay at one of the full service brand hotels in many major cities in the U.S. (“category 5”), for instance.
Still, it’s a better deal than many other Amex hotel redemptions, so if you’re strong-willed enough about using your Amex points this way, it’s a good time. It’s also reasonable for topping off towards a specific award, you have almost enough Starpoints for what you want and are going to use Amex points just to make up the difference. Then, your relevant comparison is the cost to buy Starpoints.
Alternatively, this does represent an opportunity with some devaluation to transfer points from American Express Membership Rewards to airline programs that Membership Rewards doesn’t partner with. Using this promotion, 40,000 Membership Rewards points become 20,000 Starpoints. Starwood Preferred Guest has a huge range of partners. You could transfer to American AAdvantage for instance, those 20,000 points become 25,000 miles. So 40,000 Amex points to 25,000 AAdvantage miles. The Starpoints should post quickly from Amex, and Amex transfers to airlines can take awhile, in my experience they post on Wednesdays to American as long as you get the request in by Friday. 9American will allow you under normal circumstances to hold an award for 5 days, so you could do your transfers on a Friday after putting an award on hold and odds on that means you can secure your American award in advance of making these trasnfers.)
Again, not useful in most circumstances or even close to the best use of Membership Rewards points, but strategically useful for some. I bet that a lot of folks would have taken advantage of this before December 1 when American stopped counting miles from any source, including transferred-in miles, towards lifetime status.