One of the reasons Google's advertising programs are so successful is that they are measurable. A site can compute return on investment and know that for every $X spent on Google ad clicks they are generating $Y in sales.
The retailers I'm talking about may not be representative, however: they already had a strong market presence and strong brand recognition before they went online. Maybe there are businesses that have started from scratch online that did see significant revenue from adwords, but I doubt it.
For businesses that can best exploit Adwords targeting, it can be breathtakingly effective. I've dealt with specialist contractors who saw 25000% ROI on their Adwords spend. Search terms like "loan consolidation" and "car insurance" cost tens of dollars per click and are absolutely worth that much.