I've never tried it, but I've heard it usually brings in quite a lot of views but as far as subscribers who would come back it kind of depends if they advertise to the right people. It may get you a lot more views, but hard to say if it will get you subs who will keep coming back. I know like Facebook ads work if you pay to have a post advertised through a business page. My work's Facebook page usually gets like 200-300 views per post, but every once in a while corporate will let us "boost" a post and we end up getting 3000-4000 views on whatever post we advertise and then get more likes and comments on our page as well. I think it would work similarly to how well boosting on Facebook would. Or you could always make a Facebook page for your youtube and post a video and boost it on there to try it since I think it's slightly cheaper to try out since I know that works well. Sorry I can't be of more help!