Now that I'm on my G3, I noticed this problem was even more serious. Still in the AT&T store, I received an MMS, and was not able to download it. They exchanged it with another G3. Got home, same problem. It's a complete hit or miss, being sent to me from and Android or iPhone, I receive about 80% of the incoming MMS's, and the rest I will ask the person to send it to my email, or use whatsapp if that's an available option.
I have called and emailed about the problem and I've only gotten the responses that you think a "technical level 2" employee would give.