Successes such as Spirit, Opportunity, and Curiosity make Mars seem easy.
But to be realistic, we should also be mindful of the many failures along the way:
FAILURE 1962 - Mars 1(Soviet) - Communications failed en route
FAILURE 1971 - Mars 2 and Mars 3 (Soviet) - Failure upon landing.
FAILURE 1973 - Mars 6 (Soviet) - All contact lost during landing preparation.
FAILURE 1974 - Mars 7 (Soviet) - Missed the planet.
FAILURE 1988 - Phobos 1 (Soviet) - Lost en route.
FAILURE 1988 - Phobos 2 (Soviet) - Lost en route.
FAILURE 1993 - Mars Observer (U.S.A.) - Communication lost prior to orbital insertion.
FAILURE 1999 - Mars Polar Lander (U.S.A.) - Communication lost, presumed crash landing.
FAILURE 2003 - Nozomi - Japan Mars Orbiter (Japan) - Communication lost.
The above list summarizes only the failures that came
after leaving Earth orbit. For a complete list of failures and successes, see the link below. The overall success rate from launch to landing is about 50%. It gets better if you only look at the missions since the year 2000. Within that time frame, the success rate is about 80% - one way.
http://mars.nasa.gov/programmissions/missions/log/