Well, to be honst at first: I'm not quite sure if this movie is really be underestimated. But in my perception this comic-adaption don't got the focus it deserved.
I just watched the movie another time a few hours ago and I realized new philosophical and metaphysical approaches. From my point of view Watchmen is a movie that is getting better and better the more times it is being watched. If you didn't watched it yet, be aware that the rest of this entry maybe include spoilers. But I will try my best to avoid them. Either way - you should make up for watching this movie.
So why do I think this movie is unterestimated? In our time, where the Avengers (as main-part of the Marvel Cinematic Universe) or the Justice League of America (which will rise as the main-part of the DC Cinematic Universe) dominates the image of comicbooks brought to cinema, comic-adaptions like Watchmen or R.E.D. aren't in the range of vision. Don't get me wrong - I call myself some kind of MCU-fanboy.
But movies like Watchmen don't have the typical comic-stories we like to watch in cinema these days. Take the Avengers for example: Some of them are just humans. Not normal ones like you an me. But in the end they are humans: Tony Stark (Iron Man) or Natasha Romanoff (Black Widow), for example. Other members of this group are some kind of mutants like Steve Rogers (Captain America) oder Bruce Banner (Hulk). Thor is a demi-god and take place on a higher ground. All of them have at least a heroic touch, most of them more than this.
The Watchmen are all humans. They have their skills, they have their equipment (the new generation got more then the first one, of course). But they also have their very deep-going problems. Of course - "modern" superheros have their issues, too. But normally they still have their heroic aura. This aura can't be seen when you look at the members of the Watchmen. Nite Owl II (who is of course a adaption of Batman) try to get away from this hero-identity. Silk Spectre II is overshadowed by her mother. And the Comedian as well as Rorschach are far away from being a typical hero or heroic person. These "heros" have more dark sides as every "normal" Marvel or DC-superhero could have.
But of course the information I gave that all Watchmen are human beings is wrong: Dr. Manhattan is a close-to-almighty entity. Thor and Superman are introduced as some kind of demi-gods compared to humans. But compared to Dr. Manhattan they are as weak as normal human beings. This combination of human "heros" and this creature who can be really called a demi-god creates the interplay which makes the movie this kind of mesmerising because it's so different to all we know in this genre.
Fear is the theme of the plot. Fear to get old, fear to lose something we like or love, fear to die, fear about the unknown feature. This fear in combination with the rude and violent nature of the human race leads repeatedly to very dangerous situations. Maybe it's me because I'm a political scientist, but when I watch this movie I recognise different historical approaches of politics were picked up. When the other characters are talking about the Comedian I'm always reminded of the definition of the natural state, established by Thomas Hobbes.
In Germany there is a proverb: "Der Zweck heiligt die Mittel" - or in English: "The end justifies the means." It is said that this proverb was a maxim for Heinrich Himmler, a very high-ranked member of the Nazi regime. You see - the proverb is a difficult one in Germany and not connoted very positive. But it is a central part of this movie, too. As well as the other proverb "The enemy of my enemy is my friend". And this proverb is a central part of the human nature. We all know it and we can see it very often in history or our daily live. Besides these key notes there are other difficult topics taking place. For example the emotional question about why God don't prevent a innocent person from getting harmed.
The movie and its' plot leads to the question how many hurdles have to been taken to getting peace. It deals with the conflict between the individual and the community, a question every society has to handle. Plus the movie try to blurring the border between vile and righteous deeds.
Watchmen is not just an entertaining movie, in its' best moments it is a mirror of the philosophical issues which are as old as the human race itself. And that's why Watchmen isn't just a normal movie in my opinion, it's more than that.
(And yes, of course: Malin Åkerman - one of the most beautiful women alive - as a part of the cast is another plus factor for this movie.)