I took part in a raid with my guild into Molten Core tonight. I went on Gomorra who is attuned for Molten Core. My Hunter Brieanna is also attuned from when she got her Core Hound out of there.
We started off with 10 members, 9 from the guild and one from outside the guild. I took the raid leader to get attuned earlier so we could summon people in, under the assumption that there would be one more attuned person in the raid. It turned out there wasn't another attuned person so I ran another person in. Once there were 3 of us inside the instance we could summon everyone else in.
I already had the achievement but didn't really remember my way around in there, it is a big place. As we progressed through the instance we lost a person here and a person there from them dropping out. We did manage to kill all the bosses including the last boss Ragnaros, which is the one you have to kill to get the achievement.
As usual there was some whining from some people but for the most part I think everyone had a good time. There were no deaths and we were down to 5 people by the time we got to the last boss. We still got him down without a problem. Considering the majority of us were level 80 it is not surprising.