We just finished a 4D/3N round trip from SPA to Uyuni Salt Flats with Cordillera Traveller and can recommend their excursion if you are planning the same.
Lots of companies out there so it gets confusing, we picked Cordillera based on reviews here on TA. They are a little more expensive then some of the other companies mentioned but the overall experience was very good.
The vehicles were are decent Toyota Landcruisers, not new, but were in good condition, nice to see the guys giving them a good check over every morning and a full clean-up at the end of each day. Driving standard very good, no issues there.
Driver/Guide had no English as we were told beforehand, but he was very patient with our basic spanish and explained things nice and slow. We had 17 people in 3 vehicles so it's a good idea to have at least 1 good spanish speaker in each car if possible, everyone will have a better experience this way. You'll all be transported to the Bolivian border in a minibus for an hour first, so make friends quick and try and arrange it!
Accommodation was basic, 6-bed dorm, no shower first night, double rooms with shower second night, and 4-bed and 2-bed rooms with shower third night. Food is not gourmet but acceptable. Take 2 litres of water per person for the first day (and snacks if needed), after that shops are available each evening to resupply. Take toilet roll.
We were told it gets very cold on the first night (4200m) so take a sleeping bag, they rent them also but had none left for us, so we didn't have one but in actually fact it was really warm with the 3 blankets supplied.
You'll need 150 Bolivianos pp for park entry, plus any spending money you think you'll need. We were told that at one accommodation showers were $B10 but were never charged? Also told we'd need $B15 pp for an exit fee, but again we paid nothing? But maybe best to keep a little spare in case we just got lucky!
After visiting the salt flats on day 3 you spend 2 hours in Uyuni at your leisure, bags are stored at the Cordillera office, and unfortunately one of our group appears to have had their phone stolen from their bag whilst there so be careful with your stuff! A disappointing end unfortunately.
All in all, a very well run excursion and a fantastic experience. The Salar is an amazing place, not to mention all the other lagoons, rock formations, thermal pools, geysers you'll see along the way. Happy to recommend Cordillera Traveller.