According to the Associated Press:
Scientists were jubilant over the success on a planet where two of every three lander missions have produced nothing but space junk.
According to ANDREW BRIDGES, AP Science Writer:
About two-thirds of all missions sent to the surface of Mars have failed.
According to the Arizona Republic:
Success is hardly a given, ASU scientists say. About two-thirds of the 33 NASA missions to Mars have failed.
The first two are very misleading statements, and the third is just dead wrong. JPL's list of past Mars missions shows four landers/rovers, not including MER. The Planetary Society has a table of past Mars missions. In that list, NASA has four past Mars landers, plus the current two. Even in the AP's own list, NASA's only failed lander is the Mars Polar Lander, which included two probes. Viking 1 and Viking 2 both consisted of an orbiter and a lander each, and both had successful landings. Mars Pathfinder and the Sojourner rover were outrageously and famously successful.
Therefore, NASA was three for four (75%) before MER, and now, four for five (80%). Even if you count the failed probes with MPL, NASA was three for six before MER. Most American high-school seniors could tell you that that success rate is 50%.
Sensationalistic reporters decided to include Soviet and Russian missions, with a combined 0 for 5 success rate. Why on earth would you want to include NASA's successes with others' failures? The fact that communism doesn't lend itself well to managing space exploration should not reflect on NASA's own ability.
There's a fantastic post on the Rocket Man Blog about the difficulty in getting to Mars, and he has a later post about the success rate, figuring about a 60% chance for MER. He also links to an article in Space Review by Jeff Foust with this key quote:
Mars has been one of the most popular destinations for missions beyond the Earth. Since 1960 the United States and the former Soviet Union have launched 34 missions to Mars: 15 by the US and 19 by Russia and the former USSR. NASA’s success rate is not too bad: nine of those 15 missions, including the Mars Global Surveyor and 2001 Mars Odyssey missions still in progress, can be considered successes. Russia’s luck has not been nearly as good: 14 of its 19 missions failed, and only one—Zond 3—can be considered a complete success; the remaining four are, at best, partial successes. Overall 20 of the 34 American and Russian Mars missions, or 59 percent, failed.
So, while the world may indeed only succeed once for every three attempted Mars landings, don't figure that NASA, and by extension, JPL, is that bad. We're at 80% right now! Don't believe everything you read in the paper or online, boy and girls. Getting to and landing on Mars is indeed quite difficult, but we're far better at it than your above-average baseball player is at getting on base. Keep your fingers crossed for Spirit and Opportunity!