I believe that more and more net users are going to SE's to look for something rather than search from a portal site or ISP site. This would mean that it would be better to place adwords in SE's if your site doesn't have a fantastic
PR.
When I use Google to search, I have no problem clicking on the sponsored links, if the site is not what I require, 'Back' I go and search some more. If the site is getting visitors but not sales, then capital will soon run out..
I have stopped my own adwords during these tricky few months of the year and already I have noticed a 40% drop in visits - this could also be seasonal as less people are generaly surfing at this time of year.