The positive aspects of the film are numerous. It's well directed in almost every facet, including some really nice compositions that may not be flashy but enhance the tone and style of the movie perfectly. The performances range from very solid to great, with Steve Carell showing that there's some range behind those wacky antics, and Abigail Breslin showing off both talent and bravery in a role that could very easily have been botched.
Where the film falters is in its shifts of both character and tone. They are overly dramatic and sudden, and are not always explicable. It feels like there may have been scenes cut out or changed around that may have explained some of the sudden shifts in character, but if that is not the case then the filmmakers simply chose pacing over consistency. That's fine, but it keeps the film from hitting ALL the right notes, and from greatness.
Of course that's a rather petty complaint, and generally the movie's great. You'll laugh. You may cry. You'll fall in love with "Olive." You'll finally understand that Greg Kinnear can act. You'll LOVE the ending, which is both brilliantly funny and socially astute. As a side note, I wonder what effect the Jon Benet-Ramsey case re-entering the news will have on Little Miss Sunshine. Will it increase interest or make people shy away?
Anyway go see the movie, not just because it's good but as a way of support intelligent independent film production in this country, and because if you don't you're probably just going to waste the 2 hours watching some trashy television or talking to your boring friends, with whom you've already shared everything worthwhile you have to say. If you go to this movie with them then you'll have something to discuss so your conversation won't be as soulsuckingly pointless.
P.S. One gag features iconoclast's ideal magazine, a gay porn rag called "Buns And Ammo." That ALONE should be enough to get you to the theater.