There are two portions of the mission that fascinate me the most; one is the different types of soil that we will find at the two MER landing sites, the other are the rovers themselves. I am a geologist by trade, and by taking a close look at the soils, we can understand the recent (and not so recent) history of Mars. But, I have to admit that the rovers fascinate me. Colleagues here at JPL and I have been working on making the rovers do more autonomous science. By making the rovers "smarter," they will drive further and more science data will be collected.
The most fascinating thing to me about the MER mission is the robotic exploration of the surface of Mars. Rovers provide a window for scientists to view and explore the surface of another planet. During the Mars Pathfinder mission, on landing day, when the first images were coming back to Earth, I had to stop and catch my breath realizing that I was seeing an area close-up that no other human (other than our science team) had ever seen. All the people that had lived on the planet Earth since the dawn of man had not seen these rocks and features. It was a very exciting and humbling experience. I look forward to seeing the first views from Gusev Crater and Meridiani Planum.
Being an Investigation Scientist of a mission is a very challenging job. When a scientist goes through college, by the time he/she gets their PhD, they are usually focused on one specific field or topic. But for a JPL Investigation Scientist, we are the interface between the engineers who build the spacecraft and scientist who want their instrument data for their individual research. This means we have to know all aspects of the hardware, software, operations, and the science. I would compare it to being a baseball umpire in a game where the players on each team speak different languages. Although you know the general rules of the game, you spend a lot of your time learning and understanding each teams cultural point of view (e.g. there is a difference between a baseball game in the USA and a baseball game in Japan). For MER, the more the Investigation Scientist knows about each aspect of the mission and his/her instrument, the easier the job will be.