I've seen "O", "KA", and "Allegria".
If you never seen one, I think it's worth the money completely.
KA (new at MGM Grand) - Very Creative (reminded me of Final Fantasy VII) - still waiting on their soundtrack.
Allegria (traveling around) - They do many difficult tricks. What you see on the commercial is pretty much it.
O (at Bellagio) - They do similar things like Allegria, but with water.
All 3 are good, but I liked KA the most. Great music for all Cirque de Soleil.