I had an S5 and it would no longer charge, so I upgraded to the S9 a month ago. Love it!
I also use StraightTalk and have never had a problem. Bought the S9 unlocked from Amazon and just transferred my number to it. Used Samsung transfer utility and was up and running quickly.
I was on StraightTalk, now on Visible which I like (unlimited everything and tethering for $40/mo).
I transferred my phone number to Google Voice a while back, so I never need to transfer my number anymore. I can switch whenever I want and just register my new phone with google, and no one ever knows I made a change.
So full disclosure, I have tried just about every MVNO. StraightTalk, Cricket, H20Wireless, NetZero, Boost, GoogleFi, Page Plus, Ting, Republic Wireless, Total Wireless.
My favorites so far: Cricket for ATT, Visible for Verizon (unless you need 3G, then H20), and for Sprint/T-Mobile they are all equally crap