I use it anytime I can.
Places that I've tried:
Safeway - Works flawlessly. A customer standing in line behind me thought they started accepting Apple Pay and asked the cashier. I told him it was Samsung Pay as the cashier had no clue on either technology.
Target - Interestingly worked in one location, but I received an error on a reader with EMV enabled
Nation's - I normally don't try Samsung Pay on cashier-facing terminals, but since it was within reach, I gave it a shot. The cashier/manager had to punch in the charge and card type (credit/AMEX), but it worked flawlessly afterwards.
Chick-Fil-A - Works flawlessly. One cashier was intrigued that their readers accepted the payment.
Costco - Works flawlessly. One of the cashiers exchanged looks with the bagger. I'm sure she thought I was doing voodoo magic.
CVS - Works flawlessly. The cashier started mentioning how he didn't think the terminal would accept a wireless payment, but the terminal beeped and the payment went through his sentence. Needless to say, he was amazed.
Local bike shop - The first time I used it, it crashed the system. The store owner was intrigued in the technology because he had purchased an EMV terminal without NFC capability, so he was more than willing to let me try again. The payment went without a hitch the second time around.
Best Buy - Their terminal had EMV enabled, and using Samsung Pay prompted something on the cashier's screen. I think if he had went through with the prompt it would have worked. But instead of inconvenience the both of us, I just pulled out the credit card afterwards.
I had no luck getting it to work in terminals that required you to insert your card, as I think the magnetic reader is too deep into the machine. Examples of these terminals are fuel stations and in my area, BART/metro station fare machines. Surprisingly, parking meters work just fine with Samsung Pay.
With the current implementation of EMV, Samsung/Google/Apple Pay seem to be MUCH quicker in processing payments. The EMV chip reader takes roughly 10-15 seconds which is ridiculous. I read that this might have to do with the lack of training (terminals are incorrectly set up, and are processing each payment as a whole new transaction versus being batched up), but from what I'm seeing, mobile payments are the way to go-- not just for tokenization but for speed.
All in all, I have an overwhelmingly positive experience using Samsung Pay. Once they get loyalty cards and more banks on board (which the latter is supposedly happening this month), it's going to be a knockout winner.