Wife and I just did it this past Jan. We loved the Amazon and the small ship experience but as a disclaimer, we are travel agents and probably wouldn't recommend it to customers. We did it because we haved cruised to so many other places that we do like to add exotic locales to our list. One of the main things we like to see though is a measurement the industry uses on itself; did you receive a good value for the price paid? Add the price of shots/malaria pills, plus expensive Brazilian visa to the price paid for the cruise, air and we just feel there are so many better places to spend that much money.
This is just our opinion and if someone is in the same boat we were (pun intended) and wanted to do it, I would just make sure they fully informed. Keep in mind, we enjoyed the itinerary and the ship, just a lot of hoops to jump through that amounted to a large outlay.
Hope some other folks chime in and you can always use your search engine to locate some further input.