How does the alternate system work?

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Here's my understanding, but maybe someone who's actually been an alternate can chime in:

When a player busts out of the tournament, he is replaced by the first alternate, who comes in with the starting number of chips.

So my question is this: is it an advantage or disadvantage to being an alternate? Secondly, is it an advantage to being an early alternate or one of the last alternates?

Do they set a time limit on when alternates can come into the tournament? Are alternates guaranteed to get into the event?