You should read James P. Othmer's article on Adage When I Knew Advertising Had Completely Changed. If you haven't already, check out his book Adland: Searching for the Meaning of Life on a Branded Planet (excerpt).
Here's what happened: In 20 years I went from earnest, wide-eyed junior copywriter to big agency golden boy to disillusioned, bitter, corporate burnout, then, briefly, back to golden boy, then to capable veteran and finally back to corporate burnout, but this time without the bitterness or disillusionment. Because really, there is no reason for a rational adult to be disillusioned with advertising. With medicine, or art, or the Peace Corps, maybe. But saying you're disillusioned with advertising is like saying you're disillusioned with politics or the porn industry.
What did we expect, fulfillment?