So, the core game is OK. It feels rushed and the original ending was incohherent, but it is still a fun game. However, when you factor in the Leviathan, From Ashes, and the Extended Cut DLC, the story makes much more sense, the ending makes more sense, and the overall experience goes from average to excellent in my book. The Citadel and Omega DLCs feel like bonus content to me, but the extra story and info presented in Leviathan, From Ashes, and the Extended Cut seem critical to the overall game. I know the extended cut was a knee jerk reaction to the poor reception of the original ending, but I really think the original ending would have made more sense had From Ashes and Leviathan been a part of the core game. I suspect that they might have been originally, but that Bioware took them out to sell has DLC. If this is the case, I think Bioware should have learned a hard lesson after the ending backlash they received.
So, in my opinion, if you want to get the best experience from the game, play it with the From Ashes, Leviathan, and Extended Cut DLCs installed. Regardless of what DLC you have, this game is awesome. The whole series is a must play for any real sci-fi junky.