It's not that balconies are impossible to make look good. It's just that it's hard.
If you tack them on the side of a building with heavy-looking materials, they're extremely garish, offensively ugly:
from google street view
If you recess them into the facade and use lighter materials (ie railings rather than walls), they blend a lot better. This method makes for perfectly nice and inoffensive background buildings that most people don't think of as either specifically ugly nor specifically beautiful.
from google street view
You can use that same method with a glass building and the effect is similar. Totally acceptable and appropriate for most buildings. Blends into the background.
from google street view
Or you can use them as a sculptural element themselves, to make a unique landmark. These buildings will be controversial; some people will love them and others will hate them. They're less safe but more artsy than method 2. Steely Dan posted good examples above, but there are plenty around the world.
HERE is someone's Pinterest board for sculptural skyscraper balconies, with an example below.
from jiram bernardo on flickr