Jason Fried at 37signals has spoken a few times on how little business they got from adwords (might still be the case). I remember comments from him 2+ years ago, that their adwords account was just kind of forgotten about.
Since a very tiny minority of their customers ever come from adwords now, I wonder how much smaller that would be if it was just a signup form and a few bullets of what Basecamp was going to be.
The point is, if Jason Fried had created Google adwords ads 3-4 years ago about what Basecamp was going to be before they started even putting a couple weeks into it, it sounds like he would have had a false negative.
Don't get me wrong though. I'm pointing to more about the problem with relying on adwords than the overall philosophy of the method.
Afterall, 37signals uses their blog to do this technique instead of adwords. They would leak out stuff about CRM and Sunrise many months before they ever finally had Highrise out for sale. I'm sure they got tons of great feedback on their blog and people clamoring for "yes give me simple crm!" as a strong positive signal that they were on the right path of a lucrative product.
So yes, get the minimum done and start testing on it. Just don't solely rely on taking a Google Adwords ad out though.
I recently was on a panel at the University of Illinois talking about entrepreneurship and Inkling. I was with Keith Schacht, one of the original founders of Inventables, and now he's doing craftedfun.com. He 's also religious about testing early concepts. I love a story he had about how he had an idea about some kind of mobile application the employees at a bar would use.
So right then and there he drew some stuff on paper, then went to 3 local bars near his house and asked to talk with a manager. He then bugged them each for a few minutes about his drawings on paper. And from those conversations he decided not to execute on those ideas. Sure this took balls, and he's crazy like that :)
But we all could be doing this more. And probably know someone who we could probably be bugging about something we drew on paper who could reasonably be a future customer, before we commit too much of our precious time on code and marketing.