Like I said. It is just one element among many others that has the potential to help your web site, but doesn't guarantee it's success. Having strong SEO or using adwords might put you on the first page of Google, but that doesn't guarantee sales. Having a great looking website may entice people to look around, but that doesn't guarantee sales. Having a well organized and functional site may allow people to browse through your site with ease, but doesn't guarantee sales.
There are no 100% guarantees. All these elements working together may make one incredible site but there are never guarantees that you will become rich just because you have a website. Use your website as a marketing tool, but don't ever rely solely on it. Taking care of your customers, taking pride in your work, doing the best you can will guarantee success, the website is just a tool. strong SEO is just a tool, a great looking functional layout is just a tool.
I like to think of a website as a store front. Ranking on search engines is the same as your storefronts location. Your website being harder to find on search engines is similar to your store being harder to locate in your city. This may be an obstacle to some, yet many business can succeed without being located in the center of main street.
Having an attractive an easy to navigate website, is like having an attractive storefront. It will definitely help "seal the deal" but having an attractive storefront doesn't mean the that shop does great work or has has great service.
Anyway, that is my opinion on the subject. Adwords is just a tool, just like a website. It may work for some it may not for others, the best way to know for certain is to try it and see for yourself.