There are a few different companies that offer similar cruises up the Mekong, but we had sailed with Viking River Cruises in the past and were happy with them. I booked the April sailing, the last of the season because we could not sail earlier. This meant that we had to deal with the summer heat, which made the trip uncomfortable at times, but still well worthwhile.
We took the 3 day Bangkok pre cruise extension and the 2 day Halong Bay post cruise extension. I would recommend both if you have the time, since the 12 hour difference in time from North America causes some jet lag that needs to be overcome. Both places are worth a visit if you travel that far.
We enjoyed the company of 52 other passengers, most were about the same age group of active 60 and 70 year old travelers. We got along well. The ship (boat) is very nice, built for Viking by Pandow, and is named Pandow Tonle, after the Cambodian Lake of that name. Cambodian regulations did not allow Viking to put their name on a boat until they have done business there for many years. Perhaps they will design and built a new vessel in the future, but this one was just fine, and described in detail in previous reviews.
We enjoyed the stops along the river and the activities Viking included. There were many opportunities to shop. We were always glad to get back to the air-conditioned boat because the temperatures were usually in the 90's or even 100 F, with high humidity. By the time we got to Phnom Penh some people opted to miss some trips because of this, but we did not want to miss anything. The "Killing Fields" were a must see, there were few people that did not lose a family member during this time. It really makes an impression.
So my only complaint was the heat, and we knew that it would be bad in April, and the locals said that it was worse than usual for this time of year. Our cabin was fine, but the air-conditioner was working full time and an electric fan in the cabin would have helped in the early evening.
Most of all, I was impressed with how this part of the world has recovered from the times I was there between 1965 and 1974. I could not recognize Saigon, only a few of the buildings.