I found the S5 a little too slippery without a case and feared it would slip out of my hands or, worse, out of my pocket. So I put an Incipio DualPro case on it, and the surface is rubbery and grippy enough that I feel more confident holding or pocketing the phone.
Why the Incipio? I wanted to get the case immediately, rather than waiting for mail. T-Mobile store had this case. I might have looked at the Otterbox Commuter at Best Buy, but it was all packaged up and I couldn't get a feel for it. So I now have a white phone in a gray/black case. It looks fine.