We have used Adwords the last few months. It's an integral part of our business and marketing plans for the channel.
I set the CPC to 1 cent. It nothing hits, it can be increased to 2-3 cents, then scaled back in a few hours.
Success is a fluid concept to define - what do you mean by success?
To me it's successful if I set a $10 budget per video, I get 1000 views and 500 Earned Views, about 3-6 subs, and at least 30% retention at the 50% video point. That's a fantastic result, and I would expect the video to continue rising (many have).
Doing a one off ad won't accomplish much except bump the views on the one video. Your campaign needs to be part of an overall objective.
For example, you can allocate a $100 budget over 10 days to launch a new playlist with 4 videos.
You can also allocate $20/day for the weekend for your highest retention/highest view duration video to bump up your overall channels retention and get some traffic going to your best performing video in the expectation that after the campaign is over the algorithm will recognize the good work and continue promoting it. It does!
That's just a few examples, there are many more ways a campaign can be used.
Does it work?
Yes! Very well
Remember it's in Google's best interest to make your Adwords work for you and drive traffic. They make money, you make money, they make more money when you do more campaigns. Everyone wins.