I've seen Alegria (in Philadelphia, PA) and was pleased to see the same high quality presentation and music in O. The high-divers, cross-divers, and the guy who was chillaxing while on fire were all amazing, as were the endless list of other performers that were there.
O took it to the next level with their diving pool stage/setup. Watching folks swim synchronously and never come up for air (not from this side of the pool, that is), and watching folks slowly sink into the water without a care and not come back up was as mysterious as a Cirque du Soleil show can get.
O kept my attention the entire time - recommended!