I've been using the medtronic CGM & pump for four years, and going to insulin pump support group meetings for slightly longer. The reps have been pretty honest about the CGMs being neither accurate nor precise, as one would expect from a system that's calibrated via another inaccurate system. Still, most of the group most of the time gets at least enough accuracy to determine the direction glucose is trending or if it's stable. Everyone agrees that the Dexcom CGMS beats Medtronic's CGMS on most metrics except maybe for pump integration. It seems like there's a ton of different ways to put the sensors in that affect accuracy too. Some people have scarred up areas that get bad readings, some have to change the angle of insertion to get better results, and a few of us put in the new sensor a few hours to a day before the old sensor is done because the early hours of sensor use seem less accurate. Almost everyone seems to be using the sensors at least six days and some up to three weeks. For me, ten sensors can last about 3 months.