Just this past week, I used my garmin GPSMAP76 to measure some acres.  The farmer had harvested some shelled corn, and wanted to know the acres harvested.  So, I measured it with my Garmin.  Before I went, I went to the Websurvey website, and measured some known fields on their farm.  Then I used my Garmin to measure these known areas (one was about 10 acres in size; the other was about 40 acres in size.)  In both cases, my Garmin was less than 1 acre in difference compared to the Websurvey acres.  I believe that was accurate enough for what I wanted.