In two words: highly disappointed.
But not that surprised. The PG-13 rating was a tell-tale sign.
An erotic action movie needs to be gritty and sexual, not scared of what it's trying to be in order to reach a wider audience. It's exactly why films like Sin City and 300 are both on the IMDB Top 250 list for user ratings and all the resultant copycat films are not (can anyone say The Spirit?). Sucker Punch pretty much pussied out of everything that gave it any potential for success.
Secondly, the story was pretty much shit. To go off of the above tangent, we didn't feel any real danger in the fantasy world (3rd layer) until the midpoint when they finally establish that death can carry over to the real (actually more like the 2nd layer) world. Take notes from Inception. It is clearly explained: die in the dream world, die in real life (brain-dead technically). So while the action sequences were visually stunning, there was no stress or worry or tense suspense. The viewer was disconnected from the emotion, and the story is to blame for that. Also, how the three layers of worlds connected to each other was not clearly enough defined for the ending to feel fluid. I mean were there even any other girls involved in the mental hospital world? There have to at the very least be loose threads between the worlds. A little ambiguity is fine, but we want to at least be able to guess what just happened. Lastly, if the voice over isn't amazing, get it out. It felt weak and contrived to me. Almost as bad as the dialogue.
Closing thoughts: Zack Snyder should stick to directing. Or making trailers. Cuz they were fucking sweet. Either way, let other people deal with the story.