I was going to starts Assassin's Creed 2 on the PC to see if I wanted to play through it again before heading into Brotherhood and ended up playing through the whole thing again minus most of the side missions and the feathers. I've heard folks say it doesn't have much replay value, but I loved it almost as much on the second playthrough as the first.
After that, I rushed through Brotherhood. I have to say I didn't enjoy it as much as I was hoping. A lot of the game felt tacked on like the DLC for AC 2 instead of a full new experience. There almost seemed to be daunting amount of side tasks and collections for my taste and not enough story. The story did seem to come together more after the fourth dna sequence. Before that the majority of the missions felt like they were more tutorials for side missions and mini games than anything.
I ended up running into a glitch in sequence 9 where a cutscene wouldn't load for me. I tried reloading it a few times, and checking cache in Steam. Still couldn't get it to load. I ended up watching the rest of the game on You tube so I could move on to the next game.
The Desmond story was probably my favorite thing about Brotherhood. I didn't find Ezio's story in Rome to be anywhere near as interesting as his story in the second game.
I'm on Revelations now, and enjoying it a bit more than Brotherhood, so far. I'm in DNA sequence 2 still. I'm debating rushing through this one, but then I'm hearing a lot of mixed reactions to III. So, maybe there's no hurry.