I tested 6.2.5 for days then decided to release it in its beta stages because I sold my ultra and could no longer test for bugs. Keep in mind that I 6.2.5 only has minor bugs with apps settings etc. I know for a fact that it will boot your device. The only possible way for someone to brick their ultra is if they messed up something on their part. Whether its flashing 6.2.5 for the wrong phone or not following instructions. Its not required to partition your sdcard upon installation of the rom so that's not the issue here. While it's impossible for any phone to install Megatron that is specifically made for the ultra, it is possible for any ultra to install Megatron for any other phone including the lg marquee's Megatron 6.2.5. Ever since Megatron's debuted on the ultra almost 1 year ago. Only one person has bricked their device in a situation where it was actually the developers fault.
At this point your only hope is Samsung, if your phone is less than 1 year old your still under the manufactures warranty. Whatever you do, do not tell them it was root or that you flashed a rom. As far as we know, we don't even know what root is when your talking to them. Just say you turned your phone off and it won't turn back on now. As long as there are no signs of physical damage they will repair your phone for free.