One of the questions I get asked from time to time is:
First of all, this is an involved question and experts are split over the issue. I am going to go into the pros and cons of the various positions and leave the decision for yourself. First, some background info for this not familiar with the underlying issues:
Glutamic acid is in all proteins. If you buy whey or rice protein, you can usually see on the label the L-Glutamic Acid content. The debate centers around the free from of glutamic acid, which is handled by the body completely differently than the "bound form" normally in protein. The idea is that the free form can spike blood glutamate levels much more quickly and lead to "excitotoxicity", because glutamate is an actual neurotransmitter. Increasing glutamate levels can lead to overexcitement of neurons and lead to actual neuronal death. High glutamate levels open calcium channels into neuron cells and can literally "overinflate" them to the point that they will die off shortly after. Of course, the question is at what dose this occurs.
1. MSG is Safe At Any Reasonable Dose Even for Very Young Children. This position is based on the idea that advanced animals, such as ourselves and monkeys, are largely unaffected by excitotoxins. The argument for this is based on a couple of studies on monkeys that show no hypothalmic injury even when fairly large dosages were given.  One of these studies gave a range of big doses to mice and found that the very young mice had lesions at a dose level of 0.5 g/kg of body weight for MSG and 1.0 g/kg for aspartame. In contrast, they gave 1-4 g/mg to infant monkeys and found no lesions. Their conclusion was that primates process glutamates differently than mice and thus there is likely no concern for humans either.
2. MSG is Unsafe for Young Children and Possibly Adults. Those in this camp are skeptical of the above experiements and point to several important facts:
a) MSG will spike plasma glutamate levels about fives time higher than those found in mice. 
b) A child having some soup, which is high in free glutamine, and a diet drink, which is high in free aspartic acid will see his or her plasma levels increase to 400-500 nmole/dl. This is achieved with a excitotoxin-loaded meal at about 200 mg/kg of body weight. Again, it is only the free glutamine form that counts.
c) The human/mice ratio for glutamate is about 5 and injury to the hypothalamus occurs at 60-100 nmole/l. Therefore, the plasma levels should easily achieve the hypothalamic injury level.
So, if you subscribe to this theory, you would probably want to consume less than 200 mg/kg of body weight as a bare minimum of protection.
3. MSG May Not Destroy Neurons But is Toxic Nonetheless. Some would take a middle position and point to some of the studies that are more recent that have shown toxic effects from excitotoxins such as MSG and aspartame and yet don't necessarily point to any kind of permanent neuronal damage. In other words, increasing plasma glutamate levels (or one of the other byproducts in diet drinks) may have an unforeseen or poorly understood but negative effect.
An example of this is the study showing women having "Excitotoxin Syndrome, a suite of symptoms similar to fibromyalgia. Now admittedly, this was a very small study. However, the women repeated withdrawal and reintroduction and found that MSG increased symptoms. For more information, see my link on Excitotoxin Syndrome.
Yet another example is an animal study that showed increased cancers in animals and still another a human study that shows kidney decline in diet drinkers. See my link on Monosodium Glutamate and Excitotoxins for more information.
NOTE: My position is that I get no benefit from excitotoxins and cannot think of any reason to consume them. Another way to look at this is that modern wheys, autolyzed yeast extracts and hydrolyzed proteins are all very unnatural substances where food manufacturers and processors have boiled the living crap ouf certain proteins for no good reason whatsoever. So I am protesting adding empty, non-nutritive additives to my food that may actually hurt me and my family. In addition, I believe that #3 is certainly possible as well and more study work is needed.
1) Experimental and Molecular Pathology, Oct 1975, 23(2):203 213, "Electron microscopic observations of hypothalami in neonatal rhesus monkeys (Macaca mulatta) after administration of monosodium-L-glutamate"
2) J Toxicol Environ Health, 1976 Nov, 2(2):471-80, "Hypothalamic morphology following ingestion of aspartame or MSG in the neonatal rodent and primate: a preliminary report"
3) Excitotoxins: The Taste That Kills, by Russell Blaylock, p. 37
4) Blaylock, p. 85.