I wanted to post this for several weeks now but have been too busy.
I believe every business is different and will have different results and my comments are not meant to dissuade anyone from PPC, but to merely pass along our experience.
We spent thousands of dollars on PPC over the years on various campaigns, some that were easy to track while others not so easy.
At various times, I have eliminated specific channels of ads to specific products/services and found no drop in sales.
I have worked with what started out as GoTo, then Overture and now Yahoo Search for nearly 5 or 6 years. At the end of 2006 our funds ran out and our overall sales for the first 2 1/4 months of this year are up by about 20%.
I experimented with Google Adwords with one specific product over a period of 6 months. Our sales were enough to pay for the advertising but we never made a profit.
Miva (formerly FindWhat) brought general traffic but was often plagued by fraud clicks but up until last Summer, their Miva's credit, they always credited our account back. Last summer we experienced about a 75% increase in clicks beginning in July. I investigated before filing a complaint and could clearly see a pattern of hours and keyword phrases that were being hit.
I complained citing my investigation findings and after 1 week I was told they could not identify the clicks as fraudulent. Needless to say since their clicks were for general site traffic, when our funds ran out, I discontinued advertising with them.
I do expect that we will experiment once again with PPC but do it with business targeted engines or sites like Business.com or BusinessWeek.com.
We have never done any advertisements with MSN.com but have been considering it, however, I recently read some horror stories about peoples credit cards being hit by high click charges of $100 per click or more. They were quick to respond and correct it but I fear giving my Ccard info to a service that has those kind of problems.
One other suggestion I will pass along that was provided in a paid subscription service we receive. If you are considering Google Adwords, be sure to opt out of having your ads displayed on "Content" sites. Apparently this is automatically checked and you have to physically uncheck it when you set up a campaign. You are discouraged from doing so (of course) but from the investigation conducted by the publishers of the service we subscribe to, the Content displays are where the vast majority of the click fraud occurs.