Yep, at least a third of our Mars missions have failed. I think that is what makes this success so sweet - nothing was a guarantee.
We have a much better track record than the rest of the world. We've lost 5 Missions -- 1 on launch, 1 because the probe never seperated from the rocket, 1 do to math errors, and 2 were destroyed by the Martians (MCO and MPL/DS2).
Spirit was the 15th (I think) mission to Mars. That puts us at exactly 1/3 failure rate. But remember 60% of all NASA Mars failures have nothing to do with Mars.