One of the major gripes about space exploration for the past few decades involves the decreasing importance of human-based exploration of the cosmos. While many missions are currently active, none of them involve humans actually setting foot on another world. There is no doubt that for the work that is being done, having a human presence would bolster the efforts by several magnitudes. Mars rovers have travelled an impressive distance given the harsh conditions of the planet, but given that this distance happened over many years, it loses a bit of its weight. Rather than years, a human could travel the same path in a handful of days given the right conditions.
What is forgotten by many of the voices calling for human exploration is that in every case where humans have left the planet to travel to another celestial body, exploration had already been done. Unfortunately this is a single datum, but when NASA sent astronauts to the Moon, the combined efforts of the Soviets and the Americans had landed more than a handful of landers on the surface with even more orbiters carefully observing from up high. Even before the first footprint was left in the Sea of Tranquility, two groups of astronauts had gone into orbit around our ethereal neighbor and the technology that would be used for a landing had been more than rigorously tested. In terms of Mars, missions have been much less successful at landing or even getting there. The Mars Climate Orbiter and Mars Observer are recent examples of failed Mars missions.
In addition to scouting ahead, rovers also provide a quality that a human geologist cannot do with ease. While a well trained geologist is significantly better equipped to find important rocks without any sort of communications delay, humans cannot easily run chemical analyses by themselves on rocks in the field. Humans also have the pesky limitation of needing food, water, oxygen, and companionship to survive sanely; despite a few anthropomorphic cartoons, rovers need none of that and draw no sense of loneliness from being left on the surface of a body hundreds of millions of miles from home.
Despite all of the advantages of rovers, it is inevitable that someday humanity will send humans to the stars again. To the credit of the species, humans tend to get restless without any sort of a destination and this leaves us constantly exploring for the new and the unknown, of which there is plenty beyond the comfort of our home planet. The joy of sending rovers is much akin to the joy of seeing boxes of various sizes under the Christmas tree; we may get a vague idea of what is in store from the first glimpses, but the true nature of what could be found is left to the point when we actually step on to Mars.
Within the past few weeks the Mars Exploration Laboratory, Curiosity, has made truly remarkable discoveries, namely the imaging of conglomerate rocks on Mars. Over the distance covered by prior rovers, no such traces of water were found, and yet, now stark evidence of water has been found. When it comes time for humanity to step foot on Mars, it is undeniable that even more exciting discoveries are waiting to be made. Just as a week-long trek on Earth would not even come close to revealing everything there is to know about the planet, the same holds true for all of our stellar neighbors. May the stars shine brightly in your skies.