I played through Leviathan tonight and wow was I impressed. I thought it was amazing. The story was great, the pacing was nice, and there's enough variety in gameplay to keep it fresh. The parts on the mining colony were so perfectly creepy and eerie. And I loved the detective style stuff at the office.
Also loved the extended lore, but at the same time it just made me think of how crap the endings are even more so. With the knowledge you gain from Leviathan combined with the Geth/Quarian alliance, it's just absolutely cheap and assenine that you can't convince the Reapers to abandon their harvest. I don't even care if they still made Shep die doing so...you have so much information about flaws in the Reaper's logic and info about their creation that you should've been able to say "GTFO" if you had enough war assets. Leviathan just made the endings feel even more manufactured to me.
Still, just a great piece of DLC. Way better than Omega which I thought was mostly boring.