As I recently found out, they are only good if you know you will attract the audience who are really interested. Likers need to be people who are similar others, not someone in India or in high school etc. (More for Facebook Ads) You can upload a list of your own email clients, called a "custom audience" and then find people just like them, called "lookalike audience". For promoted posts, you can select a target area, demographic and target interest. If the post is helpful, relevant, and has a vibrant photo, why not do a small budget test for 10 days. You can spend as little as $20. See your results in Insights.

As Jennifer said, you can select a target area, demographic and target interest, similar to Google ads. They can be a good idea if you know that the audience you want to target are active Facebook users. I used to ignore promoted Facebook posts as they seemed irrelevant. But, lately, they appear to be more and more very well targeted. You can accomplish something for sure. But there's another side to the coin. You need to have a really good content, not something that will look spammy and turn people away. Especially nowadays when we become increasingly aware of the marketed content that's served to us witouh us asking for it.