Has anyone tried out any of those backup battery cases? Mophie doesn't make one for the S4 as of yet I believe, but I've seen the Incipio offGRID and the ExoLife S4 online. I just got the phone, so I want to look into all the options for battery life on the phone. An extended battery or a spare is the easy way to go, I know, but I was just wondering if any of these are even worth trying? Any input would be helpful. Thanks!