I've tried Google Adwords.
It is hopelessly overpriced; I've watched my competitors continue with it - heaven knows why - but long may they continue.
A friend of mine searched for my site - he didn't even notice my Adword ad to the right of the screen.
20-30p a click might be fine if you're selling high value products with a large profit margin - but imagine selling stuff with an average sale price of £20.
Pointless.
What *does* appear to affect my sales are the "top-line" ads that people place above the listings - once again for an absurd price.
I think Google have sold out here - they claim not to use ads in the listings - hell, to an normal visitor they are at the top of the listings.
My ad gets ranked higher than somebody willing to pay more because I get clicked more because my ad is actually on topic, while the guys willing to spray money around are being sneaky and deceitful.
I think Ad Words Select is a tremendous program that works dramatically in the favor of small businesses with targeted markets. In an area like "books" Amazon can bury everybody on Overture in every subcategory but on Google Amazon couldn't bury a company called something like "Film Noir Booksellers" on a search for "film noir books". The better, more specific, more on topic listing *would* be seen and would get clicks.
[Bracket] your keywords, put in a bid one penny above the minimum, and actually have a truly good, on-topic ad, and ad words is a great program.
I'm guess that people who aren't happy with ad words select aren't using it properly or are kidding themselves on their appropriateness for the keywords.