We chose this trip solely for its itinerary (specifically, Amazon, Rio and Buenos Aires) which has proven to be quite exciting.
The ship exceeded our expectations (this was our first sail on Viking after cruising with Sliversea, Oceania, Celebrity and others).
Viking Sun is a beautifully appointed ship with exemplary design (it is also very new: delivered on 25 September 2017). Plenty of space for all 1,000 or so passengers. We experienced no line-ups anywhere, lounge chairs were always available (no chair-hogging by the pool!). Two pools reduced the pool overcrowding: one with retractable roof for cooler/rainy days and smaller Infinity pool at the very back of the ship (marvelous views of sunsets from there).
Slightly smaller than on other ships but very well designed space-wise, especially the bathroom. We felt very comfortable.