The Sprint BLU S1 requires a SIMOLW506TQ SIM Card. We have been told that Sprint is working to improve compatibility and may be allowing other Sprint SIM Cards to be compatible with the Sprint BLU S1 soon but we haven’t been unable to confirm these claims yet.
I just activated a BLU S1 as a demo model in our retail store and it feels nice to the touch. It's got a good quality build and looks like it's worth more than what we paid. I like this phone!