First, to put something to rest: this is not, as initial impressions suggested, just a glorified expansion on AC2. There are a couple of new mechanics, a new training system, a lot more side-missions/quests and, most importantly a HUGE new map to explore. I did worry that moving from three cities to one would make for a much smaller and less varied experience and, yes, Rome is not as big as the three previous cities combined and doesn't have quite the same environmental variety, but it is at least twice as big as any single previous city, with a tonne of surrounding countryside to explore, and there are even missions that take you out of Rome (little exploration to be had in these, but it's still nice to have the change of scenery).
And it still controls like Assassin's Creed. Same control-mapping, same functions and largely the same mechanics. Which, in my opinion, is a good thing...to a point.
The problem is, with exactly the same controls, comes exactly the same problem: namely, the camera. It is still too slow when traversing the environment, especially when against the clock or chasing a target. When going in a straight line - whether vertically or horizontally - it's no big deal, but any change of direction, without manually adjusting the camera (not easy when holding 'sprint'), and you'll inevitably crash into an ambling pedestrian/horse/guard, be stuck against a wall, or jump clean off a building, plummeting to a painful landing or target-losing splash.
Worse still is when the camera suddenly shifts to a fixed point when you're giving chase. This happens too often and, every time it does, the directional control shifts with it causing you to fall and fail your first attempt at almost every chase.
And the camera fairs no better in combat, often getting stuck against a wall or behind a pillar/tree. And when you do find the optimum place to watch for incoming attacks, it still zooms-in every fourth/fifth kill, only to pan out again at another bad angle.
Indeed, the camera is one of two things that make the game challenging. In fact, 'challenging' is the wrong word. AC: Brotherhood is not challenging. IF you fail at any mission during the game, it is not because the mission is difficult (even when going for 100%), but because either the camera has let you down, or no explanation has been given as to what you're supposed to be doing. This is especially true of the missions featuring The Apple.
*MILD spoiler alert*
In the latter missions of the game, you take control of The Piece of Eden (The Apple). The problem is, you're given no explanation as to how it works and you're restricted from using anything else to defend yourself. This leads to a string of frustrating and BORING climactic missions (fortunately, the final two drop The Apple entirely).
The issue of vague mission explanation is made worse by the fact that certain things are highlighted ad-nauseam (by the third time Ezio says 'Good, that will help me get back up if I fall' you'll want to throw him from something, but he'll keep on saying it), as if you can figure out a brand-new mechanic on your own, but you need telling to hold R1/RB every time you approach a lift.
There are other issues with the game that can frustrate and hamper the fun - blending doesn't always work, still no option to skip cut-scenes, the horses can no longer gallop and Ezio doesn't always grab when/what he's supposed to - but these are mostly minor.
AC: B's missions feature most of the elements from the previous two (albeit with a lot less pick-pocketing and a lot less listening-in), but these elements are predominantly used together and in a variety of ways, so the missions aren't nearly as repetitive. Leonardo's missions in particular keep things fresh.
The Borgia towers, expanded reconstruction element and Assassin recruits are also nice additions. The latter can be exploited to make things too easy at times, but that's down to how you use them and they're otherwise an interesting new element that will hopefully be further expanded upon come AC: Revelations.
If like me, you were um-ing and ah-ing between the PS3 and 360 versions, the PS3's exclusive Copernicus missions are a decent, but very brief aside, but although it looks a lot better than AC2 did on PS3, the 360 version still has slightly sharper visuals and marginally quicker loading times (installed or not), so it's down to whichever you give the higher priority (I would personally go for the sharper visuals, but that's easier to say having experienced the Copernicus missions).