The Wide Area Augmentation System is an FAA-funded project to improve the overall integrity of the GPS signal and increase position accuracy for users in North America.
The system is made up of satellites and approximately 25 ground reference stations positioned across the United States that monitor GPS data. Two master stations, located on either coast, collect data from the reference stations and create a GPS data correction message.
According to the FAA's website, testing in 9/2002 of WAAS confirmed an accuracy performance of 1-2 meters horizontal and 2-3 meters vertical throughout the majority of the continental United States and portions of Alaska.
WAAS is just one service provider that adheres to the MOPS (Minimum Operational Performance Standard) for global Satellite Based Augmentation Systems (SBAS). Eventually, there will be several services of worldwide geostationary communication satellites and ground reference stations.
Currently, enabling WAAS on your Garmin GPSMAP in regions that are not supported by ground stations may not improve accuracy, even when receiving signals from an SBAS satellite. In fact it can degrade the accuracy to less than that provided by GPS satellites alone. For this reason, when you enable WAAS on your Garmin GPS receiver, the receiver automatically uses the method that achieves the best accuracy.
For more information, go to http://gps.faa.gov/Programs/WAAS/waas.htm
This was quoted straight from page 109 in the Garmin GPS 276C manual. Where I'm located, I've disabled WAAS and the acquisition time and reliability of the GPS has improved, coincidence or not.
That's the official word from Garmin though about WAAS...by (MakMan)