In the past I would've said that that TAD was within about 5 or 10 percent of the actual market value. TAD has appropriately adjusted most of the property values downward over the past few years, but I still don't trust their numbers. I think the average variance between the actual value and TAD's values may have grown. I'd say this is particularly true for older properties, properties that haven't changed owners for many years, and properties owned by people like me who continually dispute the values assessed by the county.
As we all know, the best way to get an accurate property value is by pulling and analyzing statistics from the Multiple Listing Service (MLS) on recently sold comparable properties.