The worst variations I've seen with a Garmin handheld on 5a fields were probably about .25a given a good view of the sky (I don't consider in-cab measurements with no external antenna to be terribly reliable).  That was with trees on some edges.  Larger more open surveyed fields have given me values well under 1% off of the actual size. 

 I've got much more confidence in my Garmin handhelds than doing area with my Trimble EZGuide+.  I've seen incorrect values given by the EZG+ to be off by as much as 50 or a close to a hundred percent on fields under 5 acres.  Speaking of the Trimble, lots of other precision ag companies give great support on here, whether they're doing it officially or just have employees doing it in their spare time.  When I mentioned this area calculation problem on the Precision forum nobody working for or selling Trimble spoke up to either acknowledge the problem, tell me that my system must have a problem, or say that they've verified multiple systems on surveyed ground to be extremely accurate.  I still find that to be strange.