I promoted what I consider to be one of my best videos with AdWords, and that's what pushed me from 700 subscribers on February 20 to over 1000 by March 20. I wish I had done that a month earlier.
It ended up costing me about a dollar per subscriber, which was what I had made in AdSense revenue over the last 3 or 4 months that I was monetized, but I think it was worth it. With the added subscribers, my daily video views are now double what they were before I did the AdWords campaign. Now, I just have to wait until YouTube is finished with their review process and hopefully start making money again.
One thing I found with the ad types is that the in-stream ads gets you a lot of views, which you have to pay for, but doesn't bring in very many subscribers. The in-display ads (which just appear at the top of the recommended videos on the side of other videos) get displayed to thousands of people, and you may get a click on the ad for every couple of hundred times it's shown, but they are much more likely to subscribe because they consciously clicked on the ad because it interested them. Once again, you only pay when someone clicks on the ad, so you get more bang for your buck like that. I bid 3 or 4 cents per click and completely disabled the in-stream ads, and that's when the subscribers started rolling in. I probably wasted $150 before I figured this out, and if I had known this, the cost would've probably been 50 cents or less for each subscriber I got.
Finally, I noticed that the video that I advertised is still consistently getting several hundred views per day, even though I canceled the ad campaign a few weeks ago. Analytics shows that 60% of the views on that video are from "suggested videos", probably because of the large amount of interaction (thumbs-up and comments) it got when it was advertised.
And one more thing I forgot...when I signed up for AdWords, I got a $100 credit towards advertising, so the first $100 of ads didn't really cost me anything.
I highly recommend everyone try it out. Get your $100 ad credit, and pick a single video (make sure it's your best work). Bid low, like 3 cents per click, and disable the In-Stream ad, and set your maximum to $5.00 a day. Let it run for 20 days and see what happens. As long as you shut it down when your balance reaches $0, it won't even cost you anything.