NASA's MESSENGER probe has been orbiting Mercury for about 3 months now, since it entered orbit on March 18, 2011, being the first spacecraft to do so. In that time, the probe has taken tens of thousands of high resolution images of major features of the planet. It has taken millions of measurements of things like chemical composition, topography, and the magnetic field.
Mercury been as interesting as researchers expected, including a few surprises. NASA had a conference to today that detailed some of the things they are discovering and learning more about on Mercury. Some of the great things so far have been unprecedented surface detail that is revealing landforms unlike anything else known in the Solar System (an example can be seen in the Degas crater above, a surface composition that differs greatly from the Moon (which was thought to be analogous to Mercury, the comparison graph is below), the mapping of craters that might contain water ice or other ices, and building an understanding of energetic particle bursts in Mercury's magnetosphere (thought to be caused by interactions with the Sun).
We have learned a lot about the closest planet to the Sun in the past few months. The best part is that this mission still has another three more years! I think it's best to end with the words of the MESSENGER Principal investigator Sean Solomon of the Carnegie Institution of Washington:
"We are assembling a global overview of the nature and workings of Mercury for the first time and many of our earlier ideas are being cast aside as new observations lead to new insights. Our primary mission has another three Mercury years to run, and we can expect more surprises as our solar system's innermost planet reveals its long-held secrets."