Omega was the free one, right? And there's another one coming out soon focusing on the Collectors? I've downloaded one of the new ones (I think Omega) and I've already played through Leviathan. I'm planning on playing my 3rd round of ME over winter break, though that's definitely in the back seat compared to Assassin's Creed 3 and Far Cry 3, and possibly even WWE13.
The Collector one is free but it's an online multiplayer DLC. It doesn't really have anything to do with single player. The Omega DLC cost 1200 MS points.
I've just began my fourth playthrough on Mass Effect. And I'll be playing on insanity this time. I thought Black Ops 2 would occupy me until Christmas at least but it hasn't. Hence the new Mass Effect playthroughs. I'll probably do another of Skyrim too if the Dragonborn DLC is out soon.