I'm not always a huge fan of recent National Geographic documentaries, which can be superficial and over-dramatized. But NG's space docs are often quite good, and Expedition Mars is one of the best. It tells the story of how the two rovers, Spirit and Opportunity, have fared in their travels across the planet Mars. And it gives ample background about the project, the people and the science.
I actually saw Expedition Mars after the more-recent documentary Good Night Oppy. I was surprised to discover how much better it was, despite it's lower budget. Good Night Oppy tries so hard to reach a 'mainstream' audience - that presumably doesn't care about its subject matter - that it does a disservice to that subject matter. It's light on science, long on manufactured melodrama.
Expedition Mars doesn't skimp on the human stories behind the mission. But it spends a larger proportion of its time on technical and scientific issues. It summarizes the travels of the rovers in a more brisk and more useful way. The CG is not quite as sophisticated as in Good Night Oppy, but it's more than sufficient to let viewers visualize the various events on Mars.
Bottom line, both Expedition Mars and Good Night Oppy are worth seeing, especially if you have any interest in space exploration. But if you're only going to see one, skip Oppy and watch Expedition Mars.
