bazile
well we can measure the effectiveness of a lot of advertising because it is Direct Response - either direct mail, DRTV or these days Online. and a lot of testing is carried out to compare different creative executions and different media options for their respective yields.
The Meerkat and Gio Compario campaigns (to pick examples from another thread or two) were both shown to increase traffic to the sites and subsequent sales plus the longer term increases in Brand Awareness and Consideration.
Conventional Brand advertising is less precise but as mentioned above, test campaigns in distinct regions can produce immediate sales results, and follow up research can measure changes in Awareness, Consideration and Propensity to Buy.
As far as its precise effect on you as an individual, then you can only ask yourself if you ever make purchase decisions.
If you do, are they always based on personal experience of the product alone.
If not, what else are you using in the decision?
The Meerkat and Gio ads aren't just intended to drive people to the sites immediately, the objective is that when your renewal notice arrives, then you will go to one or more of the sites for comparison quotes