The promote button will turn your video into an ad. Depending on your video, it may bring in subs, or it may just bring views. If you're going to use it, use a trailer for a series or something that's going to catch the viewers attention as quickly as possible. (Think of how often you watch a full ad vs. how often you skip ads). The more money you enter for the campaign, the longer it's up.
If your video doesn't catch the viewers attention as quickly as possible, you'll just be left with nothing but views. Meaningless views that is.
I've used both Google Adwords (the promote button), and Virool ( a website pretty much the same thing), and have only got views. No comments, likes/dislikes, or subs.