I used it for a few months (6 or so) and had a balance upward of $3000 because I had to get some car repairs done and other expenses. It didnt trigger a FR and this was on a Golf card. I've since closed that card and got a BCE last month. I don't know how often AMEX soft pulls me, though.
I got this same letter in the mail today. I enrolled my Green card through one of the 10,000 bonus points links floating around the web from last year. Not sure if the offer is still valid but I guess I'll know in 6-8 weeks!