The only Cirque show I had seen before was KA, and that was really good. The O show was very impressive as well. The different is that (besides O is water themed) is that O doesn't try to tell a story with the performances, and the stunts while impressive and expertly choreographed, I just think KA was slightly better.
O stage is really impressive, many moving parts to the "pool" platform, and lots of crazy dives and acrobatics. Highly recommended, too. Limited view seating starts at 120$ and are actually pretty good seats. I dont think there was really any "bad" seats. Even limited view seating which starts at $120 is worth it.
Tix4Tonight usually won't have O, so you need to go to the Bellagio directly.