Here's what I consider to be outrageous clevage:
Not really sure when Christina Aguilera's boobs got so big, but when they're like half showing and there's veins everywhere I think it's a bit much (not that I don't like to look), and typically forms a bit of a negative opinion in my mind.
These Jessica Alba photos are what I consider to be more reasonable, average cleavage showings.
It really all depends on the person and the situation though I think. I'm always happy to look, so the important part is whether the person is comfortable, knows what image they're projecting, and is OK with those things.