That's a very broad question and one that is very difficult to answer.
The return you receive will be based on a number of factors. What are you using Facebook ads for? Getting people to like your Facebook page, sending users to your website?
It's like any form of advertising, you need work out if it will work for your needs. For getting facebook likes, Facebook adverts are very effective, but for generating web traffic you would want to do a comparison with other advertising solutions such as Adwords.
I've been using my $50 facebook ad coupon as a tryout over the last 6/7 days - still got about 6 days left.
Will feed back my findings, as I have been trying to get traffic to my site this way.
To date I'd summarise: initially pretty good - suddenly saw an upturn in visitors to my site, first few days. After about 5 days it all seems to have 'dried up' rather. No really new visitors hitting my site again, despite ad still running on FB....
I've just paused it for a little while, experimenting with starting again different time of day. That may help.