[friday99]

That's not a good definition of effective. The advertiser actually cares about ($ spent / additional products sold that would not have been otherwise), which is much harder to actually measure as it involves a real causal question as to whether the ads influenced a purchase. Much of remarketing spend is likely wasted on customers who were going to buy the product anyways, especially so for people making a repeat purchase.

Its a not very well kept secret of internet advertising that it's so much more "measurable" than other types of advertising, but it's not actually measuring anything useful most of the time. There is the traditional quote that "I am wasting half my advertising budget, but I can't tell which half" With internet remarketing ads the answer might very well be both ;)