Released almost a month go, the first paid DLC for Assassin's Creed: Brotherhood kind of snuck under the radar. It's a substantial addition to the game, adding a significant new memory sequence and a couple tombs to the campaign as well as modes, maps, and characters for the online multiplayer. If you're only interested in one of those components you might not think The Da Vinci Disappearance is worth your ten dollars, but I found it to be a good way to spend a few hours.
I'm not sure if you can play the memory sequence if you haven't finished the game yet, but if you have, it appears as a new waypoint on the map. Going to the waypoint unlocks the sequence, which has you reuniting with Leonardo Da Vinci in his workshop. He tells you to fetch his servant Salai playing a newly available minigame at the Thieves' Guild Headquarters, but after you find him you get attacked by a new kind of mysterious hooded enemy brandishing knives, and return the the workshop to find Leonardo missing, leaving behind a clue on how to find him. What follows is a series of missions about as long as an average story sequence from the main game, where you look for clues, track down paintings by Da Vinci, discover his location, and then tackle a temple which holds the key to yet another thing the people in the modern day need.
As was the case in the regular game, the missions are generally more fun if you go after the "full synchronization" objectives, which are unnecessary to completing the job and advancing the story, but place tighter restrictions on your play style that make you try out new things and generally provide interesting twists on what you've been doing since the second game came out a year and a half ago. Tailing someone without using the rooftops or sneaking through an area without killing anyone can be more difficult, and also more rewarding when you do something you hadn't thought of before. Not every objective is a winner (still too many revolve around not getting damaged), but the full sync stuff has generally done a good job of providing challenges to skilled players without overly punishing people who haven't mastered the series' sometimes sketchy controls.
The missions themselves provide an entertaining, The Da Vinci Code-esque mystery, as you track down the pieces of a puzzle that will lead to an important secret, one that may not be that useful to Ezio or Leonardo, but is of interesting to the people watching you in the Animus. Again, assuming you've finished the game, the DLC provides a little more on the sinister circumstances Desmond finds himself in at the end of the story, as well as giving an update on what's happening to him and what those with him are looking for. It's a nice little tease for whatever the next game in the series is, hopefully in a new setting this time. The plot within the Animus takes place before the end of the game, and gives you a little more information about a few characters as well as having a few charming moments of levity.
As far as the multiplayer side, I haven't played much of the new content (or the online at all, really), but if you can find people to play with, it's a thoroughly entertaining diversion. The new modes add on naturally to what was already established, the map is interesting, more people to stab is always cool, and it's a nice way to waste a bit of time. Personally I'm always playing new games so I don't have much time to dedicate to multiplayer, but assuming you don't expect too much out of a ten dollar DLC pack, you should have a good time with The Da Vinci Disappearance.