You might already have seen the documentary, Supersize Me, about the dangers of eating MacDonalds food in excess. The most memorable section for me is actually in the DVD special features, The Smoking Fry, an experiment to illustrate the amount of preservatives in MacDonalds items (watch the clip here).
It seems that convenience store onigiri/rice balls in Japan might not be any better, according to this article by Rick LaPointe in the Japan Times.
But it can be hard to wean off convenience foods because they are so convenient, and many people enjoy the taste. Observe the public despair over the last month or so with the disappearance from Singapore shop shelves of Maling brand luncheon meat produced in China. Imports were suspended due to a carcinogenic antibiotic, Nitrofurans, being discovered in the product (read here).