Dreamworks Animation is unfairly held to the impossibly-high standards of rival Pixar because both deal primarily in 3D Computer Animation films, despite often working in wholly different genres and styles: Pixar, with the exception of the breezy (and forgetable) "Cars," works in big-scale whimsical fantasy-dramas that happen to be cartoons; while DWA, with the exception of the "Ice Age" films, focuses on lightweight high-concept pop comedy... it's like comparing The Three Stooges to The Seventh Seal because they're both in black and white. Basically, it's not fair to ask "Monsters vs. Aliens" to live up to, say, "The Incredibles" when they have such wildly different goals and DNA.
Which isn't to say that MvA is a great film, just a decent one with a few moments of real joy. It's fast and enjoyable enough, but hovering at the margins are scraps of in-jokey movie-geek-mana that suggest a potentially more interesting concept watered-down a bit to fit in DWA's expected quota of gags and pop-culture references. But it's "good enough," overall.
The idea is that the government has been maintaining a facility to house various "monsters" over the years, all of whom are unsubtle references to iconic 50s movie-monsters: Missing Link (read: Gillman aka Creature From The Black Lagoon,) Bob (read: The Blob,) Dr. Cockroach (read: The Fly) and Insectasaurus (read: Japanese "kaiju" monsters, though which one specifically would be a spoiler) are the mainstays; recently joined by Ginormica (read: The 50 Foot Woman) a young woman turned into a giantess after being struck by a meteor on her wedding day. Said meteor and it's giant-izing power are sought by alien baddie Galaxor, who launches an invasion of Earth which the Monsters are mobilized to counter - despite having little experience or training as to using their unique abilities for combat; particularly Ginormica (real name Susan) who just wants to earn freedom to return to a normal life that she (somewhat unrealistically) hopes is still waiting for her.
As is usually the case with these things, it's more fun when the Monsters are hanging out bouncing their personas off one another than it is when all the sturm und drang gets going, but it's all impressively mounted and never wears out it's welcome. The "spectacle" part is helped ALOT by the presence of Insectasaurus, a skyscraper-sized irradiated fuzzy-bug with the mental state of a baby puppy (the army leads him around via a blinking light dangled from a helicopter) the damn thing is too fun for words. Reese Witherspoon has the mostly-thankless task of playing Ginormica as the "straight" role to the craziness around her, but she makes it work.
It's not QUITE as good as I'm given to imagine a movie about Gillman, Fly, 50 Foot Woman, The Blob and a Kaiju fighting aliens could be, but it's decent and the 3D gags are impressive.