This is not scientific in any way, but judging from the maps I have made
overlaying my GPS track on geo-referenced satellite images (using
USAPhotoMaps and TerraServer), it is extremely accurate. This track data
was made using my Garmin 196, I think WAAS was active at the time (although
it was before the official rollout).

http://home.comcast.net/~jslew/Photomap2m.png