Picked up an S2 Skyrocket in mid-December. I suspect the problem is more AT&T than the phone, but curious as to how many have had any or all of these issues:
MMS images (at any resolution the phone's camera can produce) cannot be sent to most phones. An error message "Server response error" appears and the message is marked as Failed. Those same phones can send you MMS images just fine. I had a chat session with an AT&T rep during which she had me remove the sim card and cycle the power. This worked once, and never again. E-mail, uploading to social media sites, etc. works fine.
Photos and YouTube videos won't load on the "4G" network...I have to switch to WiFi (I am thus far thoroughly unimpressed with AT&T's network performance)
3GP video doesn't play at all. All you get is a blank screen (or sometimes the first frame of video) and the sound.
Re: Galaxy S2 Skyrocket issues with MMS, 3GP video
I have a Skyrocket sitting here as well, however, I'm not experiencing the issues your describing. It could be that the SIM card could be the issue. Try putting in a different SIM card, they are provided for free at any AT&T corporate store for AT&T customers and see if that helps.
If the issues continue, it could be a hardware problem on the device. If you have the mobile insurance for your device, you can give Customer Care a call, who will be able to help exchange the device.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.