After this I'll play Revelations, it's the same engine/story/character so I hope it's again like Brotherhood but with some things added.
Can't wait for AC 3 now, very impressed with the series so far, didn't know it was this good.
Necrobump aside I've now played Revelations and I actually quite liked it.
It's really just Brotherhood 2 but the storytelling is better. I didn't like Altair at all in the first Assassin's Creed but they really made him likeable in Revelations, where they reveal what happened to him and the Creed. Otherwise it's mostly the same. The Assassin's Guild is still there with all its trimmings (calling in assassins for help, sending them on missions, etc), platforming is the same (although there's now a hookblade to make climbing faster and you can make slightly longer jumps now), and combat is the same. They add a whole bomb making mechanic but I was able to complete pretty much every mission without them outside of the missions that required them.
But it's more cinematic, there's more to do, the story is much more relevant, and the multiplayer returns. If you're able to accept the fact that it's quite the same as Brotherhood in gameplay than you'll be able to enjoy another fantastic iteration of the series.