It normally takes 3 American Express points to transfer to 1 Starwood point with the US Membership Rewards program. Through September 22, the transfer ratio is 2:1 — a 50% bonus.
This used to be a common promo, perhaps until 4 years ago. Since then the 50% bonus offers on transfers to Starwood points have more often than not been targeted.
It’s still not a great deal in most cases, unless you have a strong need for Starwood points. (I did make a final transfer from Amex to top off my Starwood account years ago when I was booking an overwater bungalow at the then-Bora Bora Nui.)