When is MSG suitable and when is it not?
MSG does have a taste on its own - umami. ElendilTheTall says in another question:


  As you are no doubt aware, there are 5 basic tastes - salty, sweet, sour, bitter and umami. Umami is the savoury flavour of mushrooms, cheese, cured meats, and so on. MSG is essentially 'pure' umami. In other words, MSG is to umami what salt is to salty and sugar is to sweet. So if you add it to savoury dishes [...] it enhances their savouriness.


MSG improves the savoriness, therefore it can be used in savory dishes in general. 

EDIT: For example savory foods are meat, vegetables, mushrooms, cheeses (if not used for sweet dishes / desserts). I think everything which has not a focus on its sweetness [*] or its ... uhm ... neutral-ness [**] can be considered as savory. 

You might have a look this this question here on cooking.SE: What is the formal definition of savory?


[*] like fruits served as a dessert, mousse au chocolat, cookies etc. However, I would consider tomatoes, caramelized onions, paprika etc., which might be sweet but not "sugary", as savory. 
[**] pancakes, "common" bread (nothing fancy), rice, ...