Be aware that as soon as you pay off the original balance (the transfer to AMEX), you might see the 3.9% offer disappear. Have they sent you checks? Or was it online? And, definitely be aware of transfer fee's (3% is common, often with limits of a min of $5 to $10 and a max of $75 to $90. However, I've also been seeing some of my cards not capping a max). It might not be worth to 3% transfer fee's. Also, be sure it clears your old 0% before transferring back.