First of all, let me be clear that I have no complaint about Hidalgo tours, who did the actual tour. The guide, Diter, was awesome and speaks very good English. Also, the itinerary was very well organized.

Now, why I strongly recommend against to book this Viator tour? To my understanding, the company that lists this tour in Viator is Fremen tours. I think it is a middle man that mostly arranges the transportation to/from La Paz airport and connect to companies that actually do tours. I'm not sure if Fremen is rotating between the companies, but I'd recommend to directly book tours with Hidalgo and purchase flight tickets from La Paz to Uyuni by yourselves. Two reasons for this.

First, Fremen tours is not respecting customers. Their email response was often extremely slow, sometimes taking more than a week. They even confused with our tour date, Nov 1st. At one point, they sent an email to remind that our tour is Oct 1st! They also doesn't inform detailed itinerary. There left a package at our hotel, but it was just a map of Bolivia. We didn't know the detailed itinerary until the actual tour day.
From our hotel to La Paz airport, David from Fremen gave a ride with a nice car. I initially thought his speaks a good English, but he only speaks and doesn't understand even simple questions. After Uyuni tour, we caught mild cold in the middle of tired and long South America trip. On the way back, we expected David's nice car, but instead he just caught a taxi for us itinerary says private vehicle in both ways.... We asked David to ask the driver to turn on the heat, but David wasn't able to understand even this simple question. The taxi driver was driving crazy.

Second, 700 per person is way expensive. You can purchase La Paz to Uyuni round-trip ticket in less than 350. I'm pretty sure that you can book a day tour directly from companies like Hidalgo much cheaper than 350.

Tour guide was knowledgeable, courteous and friendly. He was punctual. However there was waiting in Uyuni, in the hotel, wasted time. The car transport was fair. but lunch was excellent and ride was good.
Felt the tour was filled with less than informative sites. There is much to see there. However, the two guides were very well informed and gave us a very good time. The lead guide knew so much, and his knowledge base is incredible.

The salt flats were awesome and our guide and driver were great!