What it is: a sodium salt of glutamic acid. Tells a lot doesn't it...
What it's used for: to enhance tastes in tasteless micro food products sold in market. Also some other stuff. It tastes umami, which is the fifth basic taste in addition to sweetness, sourness, bitterness, and saltiness.
Why is it bad (possibly):
It allegedly can cause brain damage among other negative health effects.
More specifically, it can in large quantities spike the glutamic acid concentration of the bloodstream. Glutamic acid, or glutamate, is an important part of cellular metabolism and also a neurotransmitter, but if the amount of it gets too high it can apparently cause cell deaths and stuff in nervous system. Normally, the brain is protected by blood-brain barrier which only lets specific chemicals to pass, but not all parts of the brain are equally protected, and the "strength" of this barrier also varies by, most importantly, age. Young children tend to have somewhat less effective blood-brain barrier than adults AFAIK, and also their brain are more sensitive in general.
Effects in experiments have been brain damage to infant mice (apparently with pretty high dosages though) and obesity to lab rats.
Yum.
Note that none of these effects on humans have been proven in any recognized research, and I suspect it will eventually fall into the same category as oat porridge. It kills you if someone shoots you with projectiles made of it with sufficient velocity, or drops a sufficient load of it at you. Or if you decide to eat a kilogram of salt, it likely kills you. Or if you eat a liver of a polar bear (Ursus maritimus) you will die on overdosage of vitamin A. Doesn't mean it's dangerous in itself...
The health hazard label MSG is commonly associated with is mostly a result of a combination of carefully constructed stuff that has created a myth of MSG being a killer chemical.
Note that I don't think there should be any sane reason to add "flavour enhancers" to a food. Even if the chemical is completely safe, it does tell something about the food itself. If it does need chemical enhancement to taste like something, I think there's a problem somewhere...
