This tour is incredible, though expensive. All the tours are really similar, changing only perhaps where you stay. We carefully chose one company only to get to the Bolivian border for breakfast (in the coldest place with the scariest toilet) and discover that the Bolivian drivers with the 4x4s just come and collect any old group, so it doesn’t make any difference who you go with. The food was basic but good and our driver only spoke Spanish, but luckily one of the other tour guests could translate.
The first day included a stop at thermal lakes, where I really suggest going for a swim. It’s cold outside (4,000m) but warm in the water. After this we saw the geysers. We saw the Laguna Verde, the Arbol de Piedra and the Dali rocks, and the Laguna Colorada, beside which was the coldest hotel I have ever had the misfortune to stay in. We slept in ski suits, me with my contact lenses so they didn’t freeze into the solution. We had to break the surface of a barrel of water to "flush" the one toilet shared by 20 people. Do not expect electricity or running water!!
We also spent one night in the new salt hotel, which was fantastic. It was an absolutely beautiful hotel, with a shower (yippee!) and cheap beer. The Salar de Uyuni and Isla Pescada were incredible. The drive back after visiting the salt-mining town was horrendous. Our driver had us stay at his sister’s (no doubt to save him money, but that was okay) and then drove almost all night to get us back in time for him to take a new group.
All in all: It was tiring, cold, and far from luxurious, but there were the absolutely amazing views (see photos). If you have more time, take it, but if not, this is the best way to see a lot of a corner of Bolivia in a short time.