They are the best friend of someone trying to keep their diet as healthful as possible. These labels have been mandated and regulated since the Nutritional Labeling and Education Act of 1990 was signed into law. This is great because it lets you know what's in the food and how those ingredients break down into calories, fat, carbohydrates, proteins and nutrients. From what I've been reading, prior to this act, labels were misleading and inaccurate.
Since the law first passed, the labels have been changed numerous times. Every change has been designed to improve the information provided to the consumer. For example, trans-fats were not on them initially, but are now listed. The labels are not perfect, though. One of the biggest problems is that the serving size is not standard. Even within one product's different varieties, the serving size might be different. (Check out the different serving sizes on the different varieties of Kraft Macaroni & Cheese.)
This difference in serving sizes allows manufacturers to make claims that are true, but misleading. Terms like Low Fat and Fat Free are based on the amount of fat per serving, so a manufacturer can just reduce the serving size until the fat content is low enough to use one of those terms. This "health framing" can cause consumers to be confused or to do many math problems in the aisles.
I did find a site that can help with the math, but it doesn't have every item you might consider purchasing. Another site, plots the nutrition of an item on a graph so you can see if it's healthy and filling at a glance. It's great, but it doesn't compare items. Health framing helps food manufacturers sell more products, so it's not likely to change any time soon. Your best bet is to be very careful when shopping and to try to avoid pre-packaged food as often as possible.