I own a Nexus 6p right now, but I had a Samsung Galaxy S6 for almost a year up until a couple weeks ago. It was a cool novelty, but definitely not necessary.
By biggest issue was that my bank where I hold my checking account isn't supported, so I couldn't use my most frequently used card with, which was a debit card. In fact, I found that very few banks non-credit-card-issuing are supported. I did have 2-3 credit cards supported, but I don't use those cards often and I didn't like needing to acquire credit card debt to use this new technology. If you use a bank like Bank of America or Citi, you won't have this problem as I believe their check debit cards are supported.
I also found the MST tech to be hit or miss. Certain common terminals like those at TJ Maxx that didn't support NFC payments, were not hospitable to MST for some reason. No matter which way I held the phone, over the slot, next to it, floating it around, it could never receive the beam. After a couple of tries you can feel the stares from people in line wondering why I don't just pull out my damn wallet. I think it's the way the swipe slot is built, which makes it too deep for MST to penetrate from above or even the side.
So using it was always a decent risk of failure, which made me less likely to use it. Granted, I am in a worse boat now with Android Pay, but the feature didn't make me hesitate to jump to the 6p, and it wasn't enough incentive for me to pay twice as much for the Galaxy S7 or S7 Edge.