Having and ending spoiled doesn't do anything too me. If the ending is good, it will still be good when I watch it, and if it is bad it will still be bad.
As for how important are endings to me, well, it all depends on how good everything has been up until the end. Though usually, if the core part of the story doesn't reach my enjoyment level, then I will never get to the end, because I will have stopped because of the bad stuff that came before.
If everything has been awesome and I loved it up until the ending, then the ending doesn't really matter all that much. If there is at least some resolution, then I am fine, doesn't matter if the ending is good or bad.
That is the reason the ending of Mass Effect 3 did nothing to me. The rest of the game was amazing awesome and then I played the bad ending. My reaction was basically:
"*Yawn*... Really, that is what everybody was foaming at the mouth about? Yeah, it was bad, but not end of the world everything ruined stuff. Wow, some people are super crazy when it comes to endings not being perfect."
The people I think are the craziest, are the people that say, "I hate the ME3 ending," *unintelligible ranting*, "Aaargh, because it was so bad, all the awesome and good stuff that came before it is now all bad and not playable."
Really, boo-hoo overreaction much?