I'll say.  I used to work for a company that used feeds from DoubleClick.Net.  It's amazing that they can trace from the time that you see an ad, through all of the clicks you made to get to the purchase site, the clicks you made on the purchase site, and what your purchase eventually was.  It's actually one of the keys to prove that screen ads worked and what the cost of the ad and the payment to the site the ad was carried by should be.