First off, the reason there are two videos is that the first one has the original music, but is of poor quality and does not show the full sequence. Where as the second shows the full sequence in better quality, but the music has been changed.
Watchmen is based on the comic books of the same name, and stands out due to Zack Snyder's dark style. The films opening credits are displayed in the form of a timeline, containing multiple clips. What's clever about the sequence is that it gives viewers a look at the history of the alternate America Watchmen is set in, helping to give viewers insight before the film has properly begun. Something very useful in a film many may find confusing. Watchmen's opening sequence also contains important parts of American history through out the Cold War era, and has them retold to include characters from the film. This helps to also give background on the characters, and the fact they have been included in major parts of American history could possibly be to show their importance.
The way the credits are displayed bold and yellow gives the sequence a comic book like feel, and combined with dark imagery of alternate America, the sequence also feels like a graphic novel. The music also plays an important role in the sequence, sounding mellow and adding to the dark tone present through out the opening and the rest of the film. One downside is it's length. Whilst not a problem in a feature length film such as Watchmen, the sequence I shall being doing has a strict time limit, and so Watchmen doesn't help in showing how I can condense all I need in an opening, when it's own clocks at nearly six minutes.
Overall the sequence is fantastic, giving a brief history of the dark alternate America the film is set in, and setting viewers up for the rest of the film.