According to Wikipedia Cesium Chloride has a relatively LOW toxicity.
The worst thing about cesium chloride is that it seems to cause a potassium deficiency which may explain some of the side effects in some who have used it (such as heart arrythmia). This is easily remedied by supplementing with potassium while on cesium treatment.
Here's some more info and some positive studies on the effectiveness and low toxicity of cesium chloride:
"Caesium chloride has a low toxicity to human and animals. Its median lethal dose (LD50) in mice is 2300 mg per kilogram of body weight for oral administration and 910 mg/kg for intravenous injection. The toxicity of CsCl is related to its ability to lower the concentration of potassium in the body and partly substitute it in biochemical processes." - http://en.wikipedia.org/wiki/Caesium_chloride#Toxicity