It varies. A lot of it has to do with the actual store/retailer. I've noticed in the beginning (I was part of the Samsung Pay beta program) no one believed it would work and would be shocked. Now, most of the chain places are fairly used to them. I hear frequently now from one cashier that it does not work only to have their coworker say that it does.

So it truly is a YMMV based on how often employees get an opportunity to see it in action. Also has to do with whether that store has upgraded their POS terminal to NFC or not. Those that have usually don't have a second thought about the phone being used.

I say nowadays 1 out of 5 in person transactions (i.e. hand phone over to cashier) still gets some sort of resistance or denial.

The phone must be close to the swipe slot where you normally slide the card through. By its design, the MST has to be within inches so it isn't readable beyond the terminal. I typically bring it right next to it and move it slowly up or down (usually start in the middle) if it doesn't pick up immediately.

I will also add that I typically don't bother at US restaurants (unless I'm doing takeout) because the server has to take your credit card and there's no way to give my phone in an unlocked state to a stranger for a period of time for them to scan (assuming they would know how in the first place). It would likely have ended the first session and the secondary retry session by the time the server could process the payment. And I simply don't want my phone out of my view and in hands of strangers for any extended period of time.