I played ME3 after the EC was released, so I had to go to Youtube and look up the pre-EC ending to get a look at how bad it was. It was indeed pretty terrible. It's basically "Choose your explosion". But with the Extended Cut, it became far more sufficient. It's still not great. Nobody really understands Synthesis and I'll admit that its a bit of a Deus ex Machina. The EC added a narrative, closure and it fixed the biggest plot holes. I'm satisfied.
Although the EC did fix many issues with the original ending, the biggest issue I still have is that it didn't take into consideration any of the decisions you made during the series. Although the way they did it, does add a certain amount of closure, but I'd rather have a more open ended finale that's meaningful rather than a neat ending that doesn't adhere to the heavy decision based narrative they built up over 3 games and a couple of DLC's.