Seems to work very well here... (Colorado Springs, CO)
FWIW, this will get better as time goes on.. the Navizon software uses a database of cellphone towers and WiFi bases that have been submitted via users with GPS units... So if no one has 'logged' your area yet, you're not going to get any results
If you have a GPS and navizon capable device (See navizon.com for details...) you can help out and get paid for it too!
Man I want to do this!