I'm giving the EC ending a wavering hand. Not really quite a resounding "meh," but I certainly think there is more that should have been done. I don't think I'm alone in saying that introducing a new character - especially one that is as influential as the Catalyst - in the last 5 minutes of the game is a Bad Idea.
Now unlike some, I don't necessarily think that the Star Child needs to be taken out, especially now that he has been rewritten into a rogue AI instead of an omniscient Being. But adding some foreshadowing would be good. Even something as subtle as having the Prothean Computer refer to the Catalyst as "he" in the same discussion where it reveals that the Catalyst is the Citadel would be an improvement. Or have the Computer explain that in their search for the Catalyst, the Protheans' analysis revealed that Keeper behavior on the Citadel during their cycle indicated a higher order of reasoning than the Keepers themselves seemed capable of. Really, anything other than "O hai! I'm Zemus the Catalyst. The Reapers are my puppets, yo."
Oh, and one minor nitpick about the EC... I see that in addition to Multi-Core Shielding, we have added some serious Plot Armor to the Normandy. Or did Harbinger just decide that the ship that carries the people that have been screwing with the Reapers' plans at every turn just wasn't worth shooting at? Maybe Joker had a Red Cross painted on the hull?