For starters, i have to admit that i'm 21 and really enjoy the books (all 3 of them). Because of that i was pretty excited to see the film, even though i know it couldn't live up to the book. Sadly, i was correct.
The acting was actually quite good, as was the casting. The only character i didn't like the casting choice, regardless of the fact that she did a fine job, was Tibby. Tibby, in the book, is described as small, meek, and undeveloped. She didn't look/act that way, really.
The stories themselves were decent. Carmen and Bridget's were almost dead on (minus the fact that they left out a vital scene in the end of Bridget's). Tibby's pretty accurate, but they left out a whole lot. Lena's was completely changed, which i didn't understand why they did.
It's a teen chick flick so you expect insane cheesy moments. And the film is full of them. The soundtrack was horrible, in my opinion, making the film even more ridiculous at times. There were laugh out loud parts that probably shouldn't have been that funny.
I think if you're a 12-15 year old girl, this movie is great for you (or if you like teen chick flicks). It covers the crucial topics addressed in the book, but lacks the depth Brashares originally had. I suggest seeing it just to see it, but don't expect too much.
Also, if you're a guy, stay away. There were maximum 10 guys in the theatre when i saw it and each one looked 1/2 dead in the end.