To say that Pacific Rim is about style over substance would grossly unfair to a great action movie. It would also be technically true, but that doesn't automatically make it bad. While the substance is light it is well used and well presented, and where that may fall short, the style more than makes up for it. This is a damn stylish movie. In short, Pacific Rim is a spectacle film in much the same vein as Independence Day, though in my opinion quite a bit better.
At it's core, Pacific Rim is about is action, action, and more action. The mechs are big, the monsters arebigger, and when they punch each other you can feel the impact. Each fight is unique and dynamic, showing off the versatility of the combatants, and showcases Guillermo del Toro's aesthetic sense beautifully. More than that though, the film manages to sell the idea of 20story mechs & monsters duking it out in a realistic setting. What makes this work is a sense of scope, and scale, and consequence to everything; all of it feels real and important. In short, the action in this movie is not just fun to watch, but legitimately exciting.
Where Pacific Rim makes it's sacrifices are in the realm of story and characters. Not to say that either is bad, but rather that they are very very simple. Giant monsters have invaded the earth, and humanity bands together to repel them with giant robots. The heroes are a group of standard issues archetypes who play their parts well, but start off fully formed with little room for personal or relational development. Basically, the story does exactly what it needs to: it tells us what's going on, why we should care, and what might happen if things go wrong. Any more would just take time away from the action.
That said, the story is not entirely bland and by the books either. The setting is well developed, and hints at a larger, well-built world; our world as it might realistically be shaped by years of constant giant monster attacks. Beyond that, what makes Pacific Rim stand out is the omission of some common tropes. America does not single-handedly save the day here; the heroes are a truly international team, and because of that the film gives a great sense of the Human race united in the common cause of it's own survival.
There's really not much else to say. Pacific Rim is a simple and fun summer blockbuster. If you've ever loved old monster movies, go see it.