I've seen and liked two other Cirque shows--Mystere and Totem--and I was really looking forward to O. However, the show was underwhelming. I suspect that the show has changed over time and has gone downhill. If you are in Vegas and haven't seen any Cirque shows before, go see Mystere instead, it's a better show for much less money (at least when I was there a few years ago).
The breakdown:
The Online Ticket Interface: This thing sucks. It won't really tell you what seats are available, it only gives you what it thinks are the best available seats in the price level you set. The available seats it selects for you changes all the time, sometimes better seats, sometimes worse seats.
Cost: Pricey. Even with the 20% internet discount, the show was still $146 (w/taxes and fees) per "A" level seat. The bulk of the seats are "A" level ($155 before taxes and fees, $180 after t&f), with the cheaper seats ($109 and $130) only available on the uppermost balcony and the fringes of the theater. There are also Golden Circle ($180) and limited view seats ($98.50).
Seats: We were in Section 104, Row M, Seats 15-16, which is on the right side of the lower level. These seats were good. I would avoid the "splash zone" seats--not so much because you'll get that wet--but because they looked way too close to get a good view.
The Stage: The moving stage is impressive--a nifty piece of engineering. It fills with water or drains rapidly. But, this probably shouldn't be the thing I remember most about the show, and it is.
The Performance: "O" has a lot of choreography and spectacle, but lacks in substance. Some of the acts were repetitive, and I must admit I was actually yawning a lot near the end. The show did not have as many acrobatic set pieces as I expected.
Some of the performances were really lost in a mess of unnecessary choreography. People were just running back and forth, or someone would lower from the sky and do nothing and then go back up. And, there were definitely SLOW parts. The synchronized swimmers were boring. The divers, fire dancers, and some of the acrobats were okay. The show definitely lacked the "holy crap" moments.
Overall, the show is probably worth a 3-star rating for having a few good acts and an impressive stage.