MMS isn't supported and still silently fails (which is annoying). I just tell people to 'text my email addresss' which works, or to use something else (like facebook/snapchat).
http://googlevoiceblog.blogspot.com/2011/10/first-steps-towa...
When I get MMS from AT&T or Verizon, then I don't get anything but from Sprint customers, I get the notification and its in my gmail account right away. Probably due to the integration with Sprint and Google Voice. http://www.google.com/googlevoice/sprint/.