Having slated the Dexcom One, which was only any use as a trend device. No calibration and no two days offset the same... on a 3 month sample it was dreadful.
I've had better luck with the G7, bought 3 to try, 1 failed on day 8 (just drifted down to zero and never came back), but replaced with a new one and so far the G7 has been pretty accurate. On my 5th sensor and only one has required calibration. I made sure that the G7 was reading a straight line and then took 2 finger reading 15 minutes apart which were reading the same. Calibrated the sensor and it gave decent readings for the rest of it's life. I've noticed it's a bit out of sorts for the first 6 or so hours but otherwise has been fine.