I plan to play the entire series in order, now I'm halfway through Revelations, and there are a lot of differences from a game to another. @vladalexan is just now writing a post about the entire series, so you could check that one out.

And, a piece of advice, if you want to buy it, buy it on Steam, because for some dumb reason that I can't think of, Ubisoft is not selling some DLC on their own damn store.
For example, Assassin's Creed Revelations: The Lost Archive is included in the base game on Steam, but not on Ubisoft store, and can't be purchased separately either.