I have a Centura and they work fine for me. However, if your friends are using older iPhones like the 3GS, 4, or 4S, they may be less likely to be able to send MMS while doing other things (talking, browsing, listening to music, etc). I had this problem with my bosses, they would send blank MMS's to me. Found out they were talking on the phone to other people while sending me the MMS screenshots of my next job. Now they hang up to send the MMS's and it works fine. Just my personal experience.
Another thing to try is dial the *22890 from your phone and let it reset its settings and restart itself. If you're having issues like not being able to send or receive texts or get Chrome to browse the internet when you know darned well you have a solid Wi-Fi or 3G connection, that seems to do the trick for me if a restart alone doesn't help.