I wanted to share my experience for those checking out this thread.
My fiancee is a US Citizen, and I'm from the UK. I did have existing travel to the US before this, so good record on leaving the country. I had taken financial documents to show financial ties to the UK + Signed employment letter from my job just incase. I flew into Boston Logan, and when at the CBP Officer, I was only asked three questions:
Why I was visiting, for how long and some of the plans I had while there. To which I answered.
I was never asked to see documents, return tickets or anything. I was welcomed in (again) and everything went smoothly! (Funnily, I get the Spanish inquisition more at Heathrow I ever get at the Port of Entry for another country!)
So long as you're polite and have a good travel history? I think you'll be fine! (Politeness is definitely key!)