Did it with the guild in Wrath on my main and then again on an alt in Cataclysm. Made a little video of it too:
If you follow the link to youtube you'll be able to find a link to a little guide on how to maximize your potential for achieving this.
My personal opinion is that it was much harder in Wrath. We had 219-226 average ilvl with 4-sets back then and we struggled for a few weeks before Algalon finally went down. In Cataclysm though, it was just a matter of getting everyone to grab gear they could use (even a few blue's here and there) and then we one-shot it...
EDIT: It should also be added that the hardest part of this achievement, by far, is getting enough interested people and actually keeping them interested for the gearing up part which will take at least a month or two.