Time, we did it to increase our chances of a sticky embryo. When our first transfer failed, my Dr said that everything looked perfect so her only explanation was maybe my embryo wasn't healthy. Our second round of stims, I transferred before knowing the results because I didn't want to lose that transfer that I already paid for, but if it came back abnormal then that would have answered my question had the cycle failed. Luckily, he came back normal, and he stuck, but if he hadn't we would have tested embryos for a FET. I also think the endo scratch tremendously helped him snuggle into my lining.
I'm on metformin until I turn 12 weeks on Wednesday. I was put on it in October, and the upset stomach died down after a few weeks for me. I take 1500mg. I started by only taking 750 for the first week and then added in the other pill. It was an easier transition for my tummy.