Through December 31, Delta is offering a 30% bonus on points transferred into a Skymiles account from American Express Membership Rewards.
This is not worth cleaning out your American Express Membership Rewards account for. Don’t dump your valuable Amex points into a less valuable Skymiles account unless you have a specific use for those Delta miles, and unless you’re certain there’s availability (and it’s possible to test that first, since Amex transfers to Delta are ‘live’ and show up in a Delta account instantly).
The other thing to bear in mind is that you’ll get the 30% bonus later, a week after the transfer takes place, so you won’t be able to use those points immediately to ticket an award.
My suggestion is to transfer the points you need for a given award, ticket it, and just view this as a bonus towards the next award. Or to top off an account with a small number of miles that you don’t need right away and incorporate the 30% towards the total you’re looking for.
But if you do plan to transfer Amex points to Delta, then you will definitely want to register for this bonus first.