I decided to compare one of those store bought A1c take home tests (ReliOn brand in this case) with a lab A1c test. After my lab blood draws, I came home and used the tester.
ReliOn: 6.5%
Lab: 6.3%
So they are reasonably accurate.
But with a CGM, my GMI was also 6.5%, so not really a good point to using a take home A1c test if you have a CGM.