Played through the Battle of Forli, it does take about an hour. And minor spoiler
you also have the ability to fly around in Leo's machine, kicking guards to your hearts content, in case you missed that trophy.
The dlc is ok, there is nothing too memorable. Some of the fighting does have larger groups of enemies than the rest of the game, but it is rather easy to knife people in the back, or parry and knife them.
I'd compare it to the uplay dungeon, it is decent, if you don't play it, you haven't missed much, but if you really want to play more ac2, it fits the bill.
The dlc does lead into the 2nd dlc, but unless that one (bonfire of the vanities) turns out to be awesome, I've probably had my fill of AC2.
That being said, the game was fantastic, and an improvement over the first (except the character models, especially the faces, look kind of ugly, maybe because I played the first on the pc). I only wish there was more uses for money later in the game, a little too easy to max stuff out, more villa upgrades would have been cool.