The main advantage of manned exploration is that you have human intelligence on the spot to deal with the unexpected, whether that's equipment failure or a scientific find. A glitch like an antenna that won't deploy might be a minor repair task rather than a mission-crippling failure. Also, a person on the spot can observe and evaluate much more efficiently than the people back at Mission Control can, and can adapt to new conditions. It also eliminates the time-lag problem. Remote-controlled vehicles like the Mars rovers have to move slowly and deliberately because their controllers on Earth see what happens several minutes after it does. Compare that to the Apollo lunar rovers leaving a roostertail of dust as they rolled along.
There's also the increased sense of achievement. We have a natural desire to explore, and sending robots to do it for us is emotionally less satisfying. The first humans on Mars will catch the attention of the world, while by that time another robot rover will hardly be noticed.
The disadvantages are that it's dangerous and a lot more expensive. You need to provide life support and a ride home, while all the robot needs is a little electric power. Mission failure means that people die, which is apt to shut down the program for a long time - look what happened with the Space Shuttle. When they lose a Mars probe, they just try to avoid that problem on the next mission.
