At this point, most of you will feel like Ferelden was your actual country, Leliana's song just brings you back.
DA:O while being on of the most ambitious RPG games for X360, it's also one that possess (in my opinion) a good amount of downloadable content.
Leliana's song will bring you back to this fantasy world, which surely, by now you must feel like your own. What changes most is that you use the lovely bard you certainly romanced in the original campaign, this will give you new insight into her story.
The characters aren't quite remarkable as Leliana, and many of them will be forgotten (which is a good thing since the game is focused on Leliana), but while this happens you'll be roaming places in Ferelden that you know, only that you will do so in the boots of another character, and under very different circumstances.
I won't risk going further, because if I do, the ones that haven't played this will be led through a grievous path of spoilers, and trust me I got enough hate mail of people threating me because I ruined their games.
Should you buy it? If you really enjoy DA:O like many, and would like to have a complete set, then by all means do so, you won't be dissapointed, 4 hours of fun await you, but if you are not so enthralled by the game (sorry guys to late for me, Andraste is as real as Jesus to me) then give it a thought, you will certainly have fun and discover new things, but the gameplay and everything else will be just the same.