It's worth delving into the small print and trying to find out what they actually measure and how they do so. The Medtronic system has been around for a while now, and as far as I know is based on "1st generation CGM". It measures glucose levels in interstitial fluid, and consequently is not completely accurate, one reason why it's not normally funded. Also can be up to around 20 minutes behind the blood glucose level, depending on where the sensor is sited and other factors. Although it seems to have settled down now, when I first had mine I might as well have been monitoring my neighbour, the sensor readings bore little relation to meter values.
I have read elsewhere that the Medtronic system is known to be the least accurate of what's currently available. But it's better than nothing, and these things always seem to progress fairly rapidly.