I got the Battle of Forli DLC and enjoyed it quite a bit. The missions are predominantly combat-based, with a little parkour action thrown in here and there. The whole thing lasted about 90 minutes or so, but then it ends really abruptly. Like, you're chasing your next target out of town and then suddenly you're back in the Animus, and you're being told that the data is corrupted. My guess is that we'll continue from that point when the next DLC comes out later this month.
The only other real extra to mention is that you can now access and fly Leonardo's flying machine whenever you want. It's located at the top of the lighthouse at Forli, and this'll allow you to get the flying kick achievement/trophy if you missed it the first time around.
For $4, it's worth getting if you enjoyed the main game.
^ What he said.