O is an absolutely magnificent show with all the bells and whistles that you'd expect from a world class Cirque. I have seen most of the cirques in Vegas and a lot around the world and O really sticks out above the rest. The acrobatics and overall storyline is wonderful and visually stunning. Make sure you book seats ahead of time as weekends can get sold out quickly. Overall, highly recommended to anyone looking to be amazed.
Pros-
Concept
Theater
Storyline
Stage
Staff
Location
Visual effects
World class performers
Cons-
A bit pricey