During ISPCON Marketing Spotlight session, we were discussing why an Adwords Campaign would work for Dial-up and Hosting, but maybe not other services. The main reason is that Dial-up and Hosting don't need pre-qualification. You can sign up the customer online and they can start using the service within 30 minutes. There's very few places for the sales process to break down. That is, as long as the Prospect can click the Ad, go to your effective Landing Page, and continue through to the sign-up form and auto-bill on a credit card, you can convert Ad Words into sales almost seamlessly. (And you can track where the process breaks down with available software packages).
That process works because Dial-Up and Hosting are ubiquitous, other services are not. The sales process is more difficult with DSL and Wireless Broadband because of the pre-qual process. DSL can't go every where and neither can wireless. This means that the sales process will not be complete. It will be a contact form instead of a sign-up form. That is a problem. It means you have to have a person contact the prospect and re-start the sales process all over. Time consuming. Not as successful.
Even T1, POTS, and VOIP have limitations with where it is available.
This is a work in progress. I'll be thinking about it and writing about it. If you have thoughts, please comment.