I was on the Amazon Clipper Cruise (the full 5 days)...., it is worthwhile- I would say its excellent.

Basically, the first 3 days of the cruise cover the rio negro and the second part (2 days) is a cruise on the mighty Amazon river itself.

In terms of spotting wildlife the 2 day portion is best (on the amazon, where it seems there is more minerals in the water and hence animals)...however, the rainforest scenery on the third day (on the rio negro) is amazing.

In terms of excursions and exploration, canoe expeditions from the mother ship take place either early in the morning or late at night (wake up every day at 5.30am)- however, this is the way to for you to actually see animal life.

Overall, I really enjoyed my Amazon cruise. The Clipper Premium is a nicely appointed riverboat (air conditioned cabins, cozy beds, great dining, professional crew) and the amazon is an interesting place to visit (especially when you have the opportunity to visit small towns where the local people live, go to school etc)

BUT if your main purpose for travel is to find wildlife, be ready for disappointment- all my fellow travellers on the cruise I was on were certain that there is a better opportunity of seeing animals/birds etc in the Pantanal, Costa Rica or somewhere else in central america- the amazon is so extensive and density of animals is not so big in comparison to the territory, and as our guide explained, the animals see you, you do not see them. It is a secret forest.

....but if you choose to participate on an Amazon river cruise because you want to experience the jungle around you, the local tribes, cruise the river and take in the scenery etc, then you'll have a great time.