I'm not seeing anything suggesting that their service is unreasonably error prone
Jean-Louis Gassee posted his experience with the service. He found that his results for something as simple as a hematocrit varied considerably from one day to the next with Theranos' tests, but stayed consistent when using a traditional lab service. If they can't get something as simple as a hematocrit right, then I have serious doubts about the accuracy of anything they do.
This matches up with my potential concerns about the service when I first read about it. The volumes of blood they use are so small, that it takes only very tiny mistakes and variances to wildly change the test results.