I have an unlocked Sprint Samsung Galaxy S5 working happily on the Cricket Wireless network. I had to use a custom APN, which you can easily find on Cricket's website. However, maybe one time per day, something automatically changes my custom APN to Sprints locked APNs. A reboot resolves it.
Additionally, I have rooted it in order to remove the Sprint apps.
I was suffering with 3G only for a couple weeks even though I had set it to automatically select the best one. Just a few minutes ago, I stumbled upon a thread that made me curious about enabling international voice and data in addition to domestic under Network Mode settings. BAM! LTE!
Good luck and enjoy your converted Sprint GS5.