It wasn't clear how significant or widespread any of the problems were early on, however. Some experts have wondered how well AT&T's network will perform with the added network load from pictures and other media sent via iPhones.

Complaints on AT&T's Facebook page ranged from taking 15 to 20 minutes to send a photo wirelessly, to having an MMS transmission halt part-way through. Others said MMS service was up but then dropped out on a later attempt.

However, despite the complaints, an AT&T spokesman said MMS was working: "We and it is," said AT&T's Mark Siegel at 5 p.m. ET, several hours after activation began. "From what we can see, it's going smoothly."

While many comments were critical of the service of the hundreds that were quickly posted just hours after the turn-on, many others on AT&T's Facebook page were also positive about the new MMS capability.

Jeff Neves in Sacramento reported the service was "working great" at about 4:40 p.m. ET, : "I'm doing my part to help CRASH the network!!"