Good for what? Depends on the product. Things to look for: high in fiber, low in sugar, decent amount of protein. Some replacement bars use tons of sugar to compensate for an otherwise bland/strange taste, not good!
I personally would never purchase anything with more than 15 grams of sugar. Also, many claim to have lots of vitamins and minerals, just be careful with this one. Like many cereals, these synthetic substances are sprayed on during processing, making them less bioavailable than the natural state they are found in in foods like raw fruits, veggies, beans....
Basically, when all of this stuff is added to food as an " after thought" our bodies do not absorb it as easily as they would from the natural food source. Ex: We will be able to absorb more of the vitamin C contained in an actual orange than in a fortified product like a meal replacement bar.
If you are asking about these bars in regards to weight loss, they are not the best choice. Weight loss, and being/eating healthy in general is a life-time process, not an event. Clearly it is not reasonable to expect a person to survive off of these replacements for the rest of their lives, healthy living starts at adopting healthy practices that are part of a life-long process. In my experience, many people who use these products to lose weight, and succeed, quickly gain that weight back as soon as they begin to eat "normally."
All in all, are these replacement substances harmful? I don't think so. I have a few in my cupboard right now. In a pinch, I know they are better for me than eating nothing at all, my first choice, definitely not. Eat as close to the source as you can. (fresh, raw veggie/fruits, whole grain bread, raws nuts/seeds....) My general answer, if they are consumed in place of a healthy balanced meal; bad. If they are consumed in place of nothing at all, or a meal high in saturated fats, low in nutrients; good.