Being a local and pretty much been able to watch almost all the Cirque shows. O is captivating, it is the only water Cirque show.
A word of advice...try to avoid the late show, if there are two shows in 1 day, try to pick the earlier one. The artists had low enegy when we watched the late show. It is a "must see" and totally worth it.
It might be a good idea to be a little bit in the middle because you will be able to see the effect of the entire show.