The Dexcom rep trained me - took him all of half an hour to go over everything and then he told me to call him if I had trouble with the sensor change. The Dexcom support is amazing -- they all seem to drink the California KoolAid...
I got the G4 in November last year and it was what prompted me to start the Omnipod system in May. Basically I could see all the hypos during and after exercise and I didn't want to stop training so I had to find a better way to turn off or turn down basal before, during and after an intense racquetball match or endurance swim. It might take a bit more than 2 weeks of CGM data to be able to rely on it for complete accuracy - but it will pick up lows for sure. For me the first couple of days with a sensor are rocky - that's probably why I leave it on longer that just a week. The second week of a sensor is generally far more accurate for me than the first.