I've used it once or twice in the past. When I was doing a review on behalf of an online shopping company (based outside of Ireland), I used Trends to match high ranking key words to the meta tags, and in one case, even the product line. The website was selling home and garden machinery, but by looking at the Trends, I noted that perhaps they should sell Garden Tools as well. They're now selling lots of garden tools, and from what I gathered from the owner when I was talking to him soon after, is that a higher incline of traffic is now swinging over from Google.
It is an effective method of gathering traffic, but it took me about four weeks of spending about 15-20 minutes working on everyday (on average).