If you were running a local business ten years ago and tried to place a yellow pages ad and faced a 1 in 20 chance of being rejected through error you wouldn’t be so cavalier about a 95% accuracy rate. The same is true today of Google Maps. The key is not how many errors are present, or how long they take to get updated, its whether those errors affect you. The important point is that with waze you are in control, not some third party. Even if waze were never to become dominant in the mapping business its presence will force map providers to try to keep up with waze’s rapid update capability.
Arguments that it will take too long to build up the map database fail to take into progress in recent similar projects. If you review A History of Wikipedia you can see the effects of a nearly exponential growth on the number of articles. Wikipedia required authors with special knowledge, in contrast anyone can contribute to this project simply by turning on the app during their normal drive.

Wikipedia Growth (source: wikipedia)
waze success will come with continued participation and with benchmark moments. Twitter had its Oprah and Ashton moments as well as US Airways 1549 in the Hudson, Wikipedia had the NPR coverage of its coverage of the July 7, 2005 London bombings, so too will waze have its moments.
Crowdsourcing really accelerated at the end of 2009 and is expected to grow exponentially though out 2010. I would direct you to some of the writing of John Winsor. In addition navigation company TomTom recognizes the value of this approach and reported their 5 millionth community correction at the end of last year.
waze currently has many elements to make it poised for rapid growth. It has a game element that promotes participation through competition and it is a location based service which are very hot right now. It could use some improvements in stability and social media connectivity, but these will come.
Most importantly, their is nothing precluding you from using Google Maps as well as waze. You can contribute to waze simply by having it turned on during your regular commute. If you need to navigate to somewhere new you can always fall back on Google Maps if you don’t trust waze. It may even be possible to have it on while navigating with Google Maps, I have not tried this yet.
In response to your suggestion that waze use a commercial source as a starting point, it is not workable. waze intends to operate their company through revenue from commercial licensing of the waze map data. It would not be possible to do that with a underlying commercial map dataset. It might however be possible for a third party to create a hybrid client that combines waze realtime data with Google Maps maps. There are free map sources available in many areas and it looks as if the waze team has made use of some of them already. In my county, San Diego, CA, for example Maps and Navigation are already at the good level.
waze brings you a few features beyond what you can get from Google Maps. It is not costing you anything. It also provides an entertaining social location based game. If you are a mapping fan it provides the opportunity for you to help create/correct the maps in your area for other people to use. But all of this becomes more effective the more people use it, therefore it is vital that we spread the word about waze and share it with people who would like to have a little more fun in their every day driving. If you create a buzz around waze in your community then many of your roads will fill in quickly. The key will be in recruiting good area managers to help keep on top of update requests and make the system useful for your community.