That's what one of the passengers on Flight 1549 said he thought as the plane went down yesterday. Of course, "This is it!" can be used in positive situations, like "This is my big opportunity!" -- but here the context was more like: We're going to crash / I'm going to die / This is the end.
Of course I know how to say "ÿste es;" that's not my question. If Spanish is your native language, what would you think in a situation like that? Is there an idiomatic expression that's so ingrained it would flash into your mind in a crisis'