I'm in the process of achieving 100% completion on all Assassin's Creed games, in an epic marathon from the first all the way up to Brotherhood, and then hopefully in Revelations and III if I can get them for Christmas.
I already have 100% in Brotherhood, and I would not go through and get it again because it was a taxing, frustrating endeavor the first time around. This time, I'll just play through the main story.