Dan C
I did the whole boat down the Amazon thing, all the way from Iquitos to Belem on the Atlantic coast and it was an absolutely amazing trip. We did it on the hammock deck (you buy a hammock and sling it on the deck…) which is the cheapest way to do it, but so long as you don’t mind slumming it for a few days (4 days IIRC from Iquitos to Manaus) it’s ok.
Manaus itself is ok, but nothing special. There are a few things to see and do (this blog mentions some) but a couple of days is probably plenty. From there you’re looking at a long old flight to Uruguay though – probably with two changes in Brasilia and Sao Paulo…
Hope this helps!
Dan