All this stuff is experimental, and Tekmira in July got a clinical hold letter from the FDA for its trial in healthy patients. Now it is approved for infected patients, but it may in fact make them worse at some stages of illness.
Once the virus has proliferated to a certain stage in the blood vessels, a cascading cytokine response may not be the optimum for the patient, because it can produce or worsen cardiovascular collapse and organ failure.
Doctors have to make very tough choices here and they face a ton of unknowns. The consensus seems to be that blood transfusions (if you have a match) from recovered patients and stuff like ZMapp might be safer, because they aren't trying to provoke the body's own response. However, it does seem likely that they don't work after a certain stage of the disease.
Duncan is now receiving brincidofovir, which the FDA just approved as an emergency measure. That's an antiviral which had been in the very early investigative stage for Ebola:
http://www.usatoday.com/story/news/nation/2014/10/06/thomas-eric-duncan-ebola-patient-dallas-hospital/16798391/
There's also a Japanese antiviral, which is unproven. But my understanding is that some supplies of that have been sent to Africa.
The fact that Duncan's getting brincidofovir must mean that he's in bad shape - it's a medical Hail Mary. But the thing is, the drug is already being used in humans against cytomegalovirus, so more is known about the safety profile, and the FDA also approved it for trials against adenovirus in immune-compromised patients. So it's something that the doctors believe might help and probably won't hurt a fragile patient.
That article discusses a bit of the risks.
The Spanish priest treated with ZMapp died. There's not any silver bullet against Ebola right now - it's all just wishful thinking. There's been a pretty high survival rate in Africa with this outbreak, so it's not clear that ANY of the experimental treatments used have helped.
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